flank (n.) Look up flank at Dictionary.com
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- (cognates: 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.) Look up flank at Dictionary.com
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.