flank (n.)youdaoicibaDictYouDict
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.
ischium (n.)youdaoicibaDictYouDict
"the seat bone," 1640s, from Latin, from Greek iskhion "hip joint," in plural, "the hips," probably from iskhi "loin," of unknown origin.
sciatica (n.)youdaoicibaDictYouDict
late 14c., from Medieval Latin sciatica, in sciatica passio "sciatic disease," fem. of sciaticus "sciatic," corruption of Latin ischiadicus "of pain in the hip," from Greek iskhiadikos, from iskhias (genitive iskhiados) "pain in the hips," from iskhion "hip joint."
ornithischianyoudaoicibaDictYouDict
"Relating to or denoting herbivorous dinosaurs of an order distinguished by having a pelvic structure resembling that of birds", Early 20th century: from modern Latin Ornithiscia, from Greek ornis, ornith- 'bird' + iskhion 'hip joint'.
coxayoudaoicibaDictYouDict
"The hip bone or hip joint", Late 17th century: from Latin, 'hip'.