elbow (n.) Look up elbow at Dictionary.com
"bend of the arm," c.1200, elbowe, from a contraction of Old English elnboga "elbow," from Proto-Germanic *elino-bugon, literally "bend of the forearm" (cognates: Middle Dutch ellenboghe, Dutch elleboog, Old High German elinbogo, German Ellenboge, Old Norse ölnbogi). For first element, see ell (n.1) "length of the forearm;" second element represented by Old English boga "bow, arch" (see bow (n.1)).

Second element related to Old English bugan "to bend" (see bow (v.)); first element from *alina "arm," from PIE *el- (1) "elbow, forearm" (see ell (n.1)). To be out at elbows (1620s) was literally to have holes in one's coat. Phrase elbow grease "hard rubbing" is attested from 1670s, from jocular sense of "the best substance for polishing furniture." Elbow-room, "room to extend one's elbows," hence, "ample room for activity," attested 1530s.
elbow (v.) Look up elbow at Dictionary.com
"thrust with the elbow," c.1600, from elbow (n.). Figurative sense is from 1863. Related: Elbowed; elbowing.