Green’s Dictionary of Slang

1. to dodge, to evade.

1864		Northrop Chronicles 50: The order was ‘Shoot every man that tries to get out,’ so Boodger and I were again flanked [HDAS].
1866		C.H. Smith Bill Arp 32: They flanked me in double quick, and though my time was not out, I was constrained to depart those coasts prematurely for fear of being a desolated victim of extortion.
1872		Schele de Vere Americanisms v 286: The term to flank, which, from the strategy of the generals, descended in the mouth of privates to very lowly [...] meanings. When the men wished to escape the attention of pickets and guards by slipping past them, they said they flanked them; drill and detail and every irksome duty was flanked, when it could be avoided by some cunning trick. Soon [...] the poor farmer was flanked out of his pig and his poultry.
1891		Rodenbough Sabre 138: The ... battle of Spotsylvania had been fought in the mean time, and I...successfully flanked it.
1943		(con. 1861-5) B.I. Wiley Lifeof Johnny Reb 48: Not to mention the Yanks who flanked instead of fighting.

2. to trick out of.

1872		see sense 1.
1879		Southern Historical Society Papers vii 394: The Government never made anything by employing these ‘rebels,’ as they invariably ‘flanked’ more than they received as pay.

3. (N.Z. prison) for two inmates to attack another.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 70/1: flank v. = top and tail.