late O.E. flanc "fleshy part of the side," from O.Fr. flanc, probably from Frank. *hlanca (cf. O.H.G. (h)lanca, M.H.G. lanke "hip joint," Ger. lenken "to bend, turn, lead"), from PIE base *qleng- "to bend." The military sense is first attested 1540s, as is the verb. Related: Flanked; flanking.