Flank

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late Old English, from Old French flanc, of Germanic origin.

wiktionary

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From Late Middle English flanc, from Late Old English flanc(“flank”), from Old French flanc, of Germanic origin, probably Frankish *hlanca, from Proto-Germanic *hlankō(“bend, curve, hip, flank”), from Proto-Germanic *hlankaz(“flexible, sleek, bendsome”), from Proto-Indo-European *kleng-(“to bend”). Akin to Old High German hlanca(“loin”), Middle Low German lanke(“hip joint”) (German lenken(“to bend, turn, lead”)), Old English hlanc(“loose, slender, flaccid, lank”). More at lank.

etymonline

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flank (n.)

late Old English flanc "flank, fleshy part of the side," from Old French flanc "hip, side," from Frankish or another Germanic source, from Proto-Germanic *hlanca- (source also of Old High German (h)lanca, Middle High German lanke "hip joint," German lenken "to bend, turn aside;" Old English hlanc "loose and empty, slender, flaccid;" Old Norse hlykkr "a bend, noose, loop"), from PIE root *kleng- "to bend, turn" (see link (n.)). Showing characteristic change of Germanic hl- to Romanic fl-. The military sense is first attested 1540s. Meaning "side" of anything is by 1620s. As an adjective, "pertaining to the flank or side," 1660s. Related: Flanked; flanking.

flank (v.)

1590s (military), "to guard the flank," also, "to menace the flank, fire sideways upon," from flank (n.). Meaning "stand or be placed at the side of" is from 1650s. Related: Flanked; flanking.