flank (n.) Look up flank at Dictionary.com
late Old English flanc "fleshy part of the side," from Old French flanc, from Frankish, from Proto-Germanic *hlanca (cognates: Old High German (h)lanca, Middle High German lanke "hip joint," German lenken "to bend, turn, lead"), from PIE root *kleng- "to bend, turn" (see link (n.)). The military sense is first attested 1540s, as is the verb. Related: Flanked; flanking.