The Collaborative International Dictionary of English v.0.48:

Arch \Arch\, n. [See Arch-, pref.]
   A chief. [Obs.]
   [1913 Webster]

         My worthy arch and patron comes to-night. --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

-arch \-arch\ [Gr. 'archo`s chief, commander, 'a`rchein to rule.
   See Arch, a.]
   A suffix meaning a ruler, as in monarch (a sole ruler).
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Arch \Arch\, v. t. [imp. & p. p. Arched ([aum]rcht); p. pr. &
   vb. n. Arching.]
   1. To cover with an arch or arches.
      [1913 Webster]

   2. To form or bend into the shape of an arch.
      [1913 Webster]

            The horse arched his neck.            --Charlesworth.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Arch \Arch\, v. i.
   To form into an arch; to curve.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Arch \Arch\ ([aum]rch), n. [F. arche, fr. LL. arca, for arcus.
   See Arc.]
   1. (Geom.) Any part of a curved line.
      [1913 Webster]

   2. (Arch.)
      (a) Usually a curved member made up of separate
          wedge-shaped solids, with the joints between them
          disposed in the direction of the radii of the curve;
          used to support the wall or other weight above an
          opening. In this sense arches are segmental, round (i.
          e., semicircular), or pointed.
      (b) A flat arch is a member constructed of stones cut into
          wedges or other shapes so as to support each other
          without rising in a curve.
          [1913 Webster]

   Note: Scientifically considered, the arch is a means of
         spanning an opening by resolving vertical pressure into
         horizontal or diagonal thrust.
         [1913 Webster]

   3. Any place covered by an arch; an archway; as, to pass into
      the arch of a bridge.
      [1913 Webster]

   4. Any curvature in the form of an arch; as, the arch of the
      aorta. "Colors of the showery arch." --Milton.
      [1913 Webster]

   Triumphal arch, a monumental structure resembling an arched
      gateway, with one or more passages, erected to commemorate
      a triumph.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Arch- \Arch-\ ([aum]rch-, except in archangel and one or two
   other words). [L. arch-, Gr. 'arch- = 'archi-. See Arch-.]
   A prefix signifying chief, as in archbuilder, archfiend.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Arch \Arch\ ([aum]rch), a. [See Arch-, pref.]
   1. Chief; eminent; greatest; principal.
      [1913 Webster]

            The most arch act of piteous massacre. --Shak.
      [1913 Webster]

   2. Cunning or sly; sportively mischievous; roguish; as, an
      arch look, word, lad.
      [1913 Webster]

            [He] spoke his request with so arch a leer.
                                                  --Tatler.
      [1913 Webster]

WordNet (r) 3.0 (2006):

arch
    adj 1: (used of behavior or attitude) characteristic of those
           who treat others with condescension [syn: arch,
           condescending, patronizing, patronising]
    2: expert in skulduggery; "an arch criminal"
    3: naughtily or annoyingly playful; "teasing and worrying with
       impish laughter"; "a wicked prank" [syn: arch, impish,
       implike, mischievous, pixilated, prankish, puckish,
       wicked]
    n 1: a curved shape in the vertical plane that spans an opening
    2: a curved bony structure supporting or enclosing organs
       (especially the inner sides of the feet)
    3: a passageway under a curved masonry construction; "they built
       a triumphal arch to memorialize their victory" [syn: arch,
       archway]
    4: (architecture) a masonry construction (usually curved) for
       spanning an opening and supporting the weight above it
    v 1: form an arch or curve; "her back arches"; "her hips curve
         nicely" [syn: arch, curve, arc]