The Collaborative International Dictionary of English v.0.48:

Flanker \Flank"er\, n.
   One who, or that which, flanks, as a skirmisher or a body of
   troops sent out upon the flanks of an army toguard a line of
   march, or a fort projecting so as to command the side of an
   assailing body.
   [1913 Webster]

         They threw out flankers, and endeavored to dislodge
         their assailants.                        --W. Irwing.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Flanker \Flank"er\, v. t. [imp. & p. p. Flankered; p. pr. &
   vb. n. Flankering.] [See Flank, v. t.]
   1. To defend by lateral fortifications. [Obs.] --Sir T.
      Herbert.
      [1913 Webster]

   2. To attack sideways. [Obs.] --Evelyn.
      [1913 Webster]

WordNet (r) 3.0 (2006):

flanker
    n 1: a back stationed wide of the scrimmage line; used as a pass
         receiver [syn: flanker back, flanker]
    2: a soldier who is a member of a detachment assigned to guard
       the flanks of a military formation