The Collaborative International Dictionary of English v.0.48:

Flank \Flank\ (fl[a^][ng]k), n. [F. flanc, prob. fr. L. flaccus
   flabby, with n inserted. Cf. Flaccid, Flanch, Flange.]
   1. The fleshy or muscular part of the side of an animal,
      between the ribs and the hip. See Illust. of Beef.
      [1913 Webster]

   2. (Mil.)
      (a) The side of an army, or of any division of an army, as
          of a brigade, regiment, or battalion; the extreme
          right or left; as, to attack an enemy in flank is to
          attack him on the side.
          [1913 Webster]

                When to right and left the front

                Divided, and to either flank retired. --Milton.
      (b) (Fort.) That part of a bastion which reaches from the
          curtain to the face, and defends the curtain, the
          flank and face of the opposite bastion; any part of a
          work defending another by a fire along the outside of
          its parapet. See Illust. of Bastion.
          [1913 Webster]

   3. (Arch.) The side of any building. --Brands.
      [1913 Webster]

   4. That part of the acting surface of a gear wheel tooth that
      lies within the pitch line.
      [1913 Webster]

   Flank attack (Mil.), an attack upon the side of an army or
      body of troops, distinguished from one upon its front or
      rear.

   Flank company (Mil.), a certain number of troops drawn up
      on the right or left of a battalion; usually grenadiers,
      light infantry, or riflemen.

   Flank defense (Fort.), protection of a work against undue
      exposure to an enemy's direct fire, by means of the fire
      from other works, sweeping the ground in its front.

   Flank en potence (Mil.), any part of the right or left wing
      formed at a projecting angle with the line.

   Flank files, the first men on the right, and the last on
      the left, of a company, battalion, etc.

   Flank march, a march made parallel or obliquely to an
      enemy's position, in order to turn it or to attack him on
      the flank.

   Flank movement, a change of march by an army, or portion of
      one, in order to turn one or both wings of the enemy, or
      to take up a new position.

   Flanks of a frontier, salient points in a national
      boundary, strengthened to protect the frontier against
      hostile incursion.

   Flank patrol, detachments acting independently of the
      column of an army, but patrolling along its flanks, to
      secure it against surprise and to observe the movements of
      the enemy.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Flank \Flank\, v. i.
   1. To border; to touch. --Bp. Butler.
      [1913 Webster]

   2. To be posted on the side.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Flank \Flank\ (fl[a^][ng]k), v. t. [imp. & p. p. Flanked
   (fl[a^][ng]kt); p. pr. & vb. n. Flanking.] [Cf. F.
   flanquer. See Flank, n., and cf. Flanker, v. t.]
   1. To stand at the flank or side of; to border upon.
      [1913 Webster]

            Stately colonnades are flanked with trees. --Pitt.
      [1913 Webster]

   2. To overlook or command the flank of; to secure or guard
      the flank of; to pass around or turn the flank of; to
      attack, or threaten to attack; the flank of.
      [1913 Webster]

WordNet (r) 3.0 (2006):

flank
    n 1: the side of military or naval formation; "they attacked the
         enemy's right flank" [syn: flank, wing]
    2: a subfigure consisting of a side of something
    3: a cut from the fleshy part of an animal's side between the
       ribs and the leg
    4: the side between ribs and hipbone
    v 1: be located at the sides of something or somebody

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "flank":
   Chateaubriand, advance against, advance upon, advocate, bank, beam,
   bear down upon, blade roast, border, breast, brisket, broadside,
   champion, cheek, chop, chuck, chuck roast, circle, circuit,
   circuiteer, circulate, circumambulate, circummigrate,
   circumnavigate, circumvent, clod, close the circle, coast,
   cold cuts, come full circle, compass, counterattack, cycle, defend,
   describe a circle, drive, edge, encircle, encompass, filet mignon,
   gas, girdle, girdle the globe, go about, go around, go round,
   go the round, guard, guard against, gyre, hand, handedness, haunch,
   hip, infiltrate, jowl, knuckle, lap, laterality, launch an attack,
   line, loin, make a circuit, many-sidedness, march against,
   march upon, mount an attack, multilaterality, open an offensive,
   orbit, outflank, planking, plate, plate piece, pot roast, profile,
   protect, push, quarter, rack, revolve, rib roast, ribs, roast,
   rolled roast, round, rump, rump roast, saddle, safeguard, screen,
   secure, shank, shield, shore, short ribs, shoulder, shoulder clod,
   side, siding, sirloin, skirt, spiral, strike, surround, temple,
   tenderloin, thrust, unilaterality, wheel