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]