From 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]
From 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]