The Collaborative International Dictionary of English v.0.48:

Fog \Fog\ (f[o^]g), n. [Cf. Scot. fog, fouge, moss, foggage rank
   grass, LL. fogagium, W. ffwg dry grass.] (Agric.)
   (a) A second growth of grass; aftergrass.
   (b) Dead or decaying grass remaining on land through the
       winter; -- called also foggage. [Prov.Eng.]
       --Halliwell.

   Note: Sometimes called, in New England, old tore. In
         Scotland, fog is a general name for moss.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fog \Fog\, v. t. [imp. & p. p. Fogged; p. pr. & vb. n.
   Fogging.]
   1. To envelop, as with fog; to befog; to overcast; to darken;
      to obscure.
      [1913 Webster]

   2. (Photog.) To render semiopaque or cloudy, as a negative
      film, by exposure to stray light, too long an exposure to
      the developer, etc.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Fog \Fog\ (f[o^]g), v. t. (Agric.)
   To pasture cattle on the fog, or aftergrass, of; to eat off
   the fog from.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fog \Fog\ (f[o^]g), v. i. [Etymol. uncertain.]
   To practice in a small or mean way; to pettifog. [Obs.]
   [1913 Webster]

         Where wouldst thou fog to get a fee?     --Dryden.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fog \Fog\, v. i. (Photog.)
   To show indistinctly or become indistinct, as the picture on
   a negative sometimes does in the process of development.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fog \Fog\ (f[o^]g), n. [Dan. sneefog snow falling thick, drift
   of snow, driving snow, cf. Icel. fok spray, snowdrift,
   fj[=u]k snowstorm, fj[=u]ka to drift.]
   1. Watery vapor condensed in the lower part of the atmosphere
      and disturbing its transparency. It differs from cloud
      only in being near the ground, and from mist in not
      approaching so nearly to fine rain. See Cloud.
      [1913 Webster]

   2. A state of mental confusion.
      [1913 Webster]

   3. (Photog.) Cloudiness or partial opacity of those parts of
      a developed film or a photograph which should be clear.
      [Webster 1913 Suppl.]

   Fog alarm, Fog bell, Fog horn, etc., a bell, horn,
      whistle or other contrivance that sounds an alarm, often
      automatically, near places of danger where visible signals
      would be hidden in thick weather.

   Fog bank, a mass of fog resting upon the sea, and
      resembling distant land.

   Fog ring, a bank of fog arranged in a circular form, --
      often seen on the coast of Newfoundland.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cloud \Cloud\ (kloud), n. [Prob. fr. AS. cl[=u]d a rock or
   hillock, the application arising from the frequent
   resemblance of clouds to rocks or hillocks in the sky or
   air.]
   1. A collection of visible vapor, or watery particles,
      suspended in the upper atmosphere.
      [1913 Webster]

            I do set my bow in the cloud.         --Gen. ix. 13.
      [1913 Webster]

   Note: A classification of clouds according to their chief
         forms was first proposed by the meteorologist Howard,
         and this is still substantially employed. The following
         varieties and subvarieties are recognized:
      (a) Cirrus. This is the most elevated of all the forms
          of clouds; is thin, long-drawn, sometimes looking like
          carded wool or hair, sometimes like a brush or room,
          sometimes in curl-like or fleecelike patches. It is
          the cat's-tail of the sailor, and the mare's-tail of
          the landsman.
      (b) Cumulus. This form appears in large masses of a
          hemispherical form, or nearly so, above, but flat
          below, one often piled above another, forming great
          clouds, common in the summer, and presenting the
          appearance of gigantic mountains crowned with snow. It
          often affords rain and thunder gusts.
      (c) Stratus. This form appears in layers or bands
          extending horizontally.
      (d) Nimbus. This form is characterized by its uniform
          gray tint and ragged edges; it covers the sky in
          seasons of continued rain, as in easterly storms, and
          is the proper rain cloud. The name is sometimes used
          to denote a raining cumulus, or cumulostratus.
      (e) Cirro-cumulus. This form consists, like the cirrus,
          of thin, broken, fleecelice clouds, but the parts are
          more or less rounded and regulary grouped. It is
          popularly called mackerel sky.
      (f) Cirro-stratus. In this form the patches of cirrus
          coalesce in long strata, between cirrus and stratus.
      (g) Cumulo-stratus. A form between cumulus and stratus,
          often assuming at the horizon a black or bluish tint.
          -- Fog, cloud, motionless, or nearly so, lying near
          or in contact with the earth's surface. -- Storm
          scud, cloud lying quite low, without form, and driven
          rapidly with the wind.
          [1913 Webster]

   2. A mass or volume of smoke, or flying dust, resembling
      vapor. "A thick cloud of incense." --Ezek. viii. 11.
      [1913 Webster]

   3. A dark vein or spot on a lighter material, as in marble;
      hence, a blemish or defect; as, a cloud upon one's
      reputation; a cloud on a title.
      [1913 Webster]

   4. That which has a dark, lowering, or threatening aspect;
      that which temporarily overshadows, obscures, or
      depresses; as, a cloud of sorrow; a cloud of war; a cloud
      upon the intellect.
      [1913 Webster]

   5. A great crowd or multitude; a vast collection. "So great a
      cloud of witnesses." --Heb. xii. 1.
      [1913 Webster]

   6. A large, loosely-knitted scarf, worn by women about the
      head.
      [1913 Webster]

   Cloud on a (or the) title (Law), a defect of title,
      usually superficial and capable of removal by release,
      decision in equity, or legislation.

   To be under a cloud, to be under suspicion or in disgrace;
      to be in disfavor.

   In the clouds, in the realm of facy and imagination; beyond
      reason; visionary.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fog
    n 1: droplets of water vapor suspended in the air near the
         ground
    2: an atmosphere in which visibility is reduced because of a
       cloud of some substance [syn: fog, fogginess, murk,
       murkiness]
    3: confusion characterized by lack of clarity [syn: daze,
       fog, haze]
    v 1: make less visible or unclear; "The stars are obscured by
         the clouds"; "the big elm tree obscures our view of the
         valley" [syn: obscure, befog, becloud, obnubilate,
         haze over, fog, cloud, mist]

Moby Thesaurus II by Grady Ward, 1.0:

191 Moby Thesaurus words for "fog":
   CAT, London fog, London special, addle, addle the wits, aerospace,
   aerosphere, aftergrass, air hole, air pocket, airspace,
   amorphousness, ball up, bamboo, becloud, bedazzle, bedim, befog,
   befuddle, befuddlement, bemist, bewilder, bewilderment, blur,
   bother, botheration, bug, bump, cane, ceiling, cereal,
   cereal plant, chaos, clabber up, cloud, cloud over, cloud up,
   confuse, confusion, corn, crosswind, dark, darken, darken over,
   darkness, daze, dazzle, defocus, deform, dim, discombobulate,
   discombobulation, discomfit, discomfiture, discompose,
   discomposure, disconcert, disconcertion, disorder, disorganization,
   disorganize, disorient, disorientation, distort, distract, disturb,
   disturbance, drisk, drizzling mist, eclipse, embarrass,
   embarrassment, empty space, encloud, enmist, entangle,
   farinaceous plant, favorable wind, film, flummox, flurry, fluster,
   flutter, fog up, fogginess, forage grass, frenzy, front,
   frost smoke, fuddle, fuddlement, fuss, fuzz, fuzziness, gauze,
   grain, graminaceous plant, grass, haze, head wind,
   high-pressure area, hole, indeterminateness, indistinctness,
   ionosphere, jetstream, jumble, lawn grass, lose resolution,
   low-pressure area, make uncertain, maze, mess, mess up, mist,
   mistiness, mix up, moider, muddle, muddlement, muddy, mumbo jumbo,
   murk, murkiness, mystification, mystify, nubilate, obfuscate,
   obfuscation, obnubilate, obscurantism, obscuration, obscure,
   obscurity, opacity, ornamental grass, overcast, overcloud,
   overshadow, oversmoke, pale, pea soup, pea-soup fog, peasouper,
   perplex, perplexity, perturb, perturbation, pocket, pother, pucker,
   put out, puzzle, raise hell, rattle, reed, roughness, ruffle,
   shade, shadow, shapelessness, shuffle, smog, smoke, soften, soup,
   space, stew, stratosphere, substratosphere, sweat, swivet,
   tail wind, throw into confusion, tizzy, tropopause, troposphere,
   trough, turbulence, unclarity, unclearness, unform, unplainness,
   unsettle, unsettlement, unshape, upset, vagueness, vapor,
   visibility, visibility zero