What is the X in X-mas?
late Old English flanc "fleshy part of the side," from Old French flanc, probably from Frankish *hlanca (cf. 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.
The side of the body between the pelvis or hip and the last rib; the side.
The section of flesh in that area.