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.
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            Stately colonnades are flanked with trees. --Pitt.
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   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.
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