elbow: [OE] Logically enough, elbow means etymologically ‘arm bend’. It comes from a prehistoric West and North Germanīc *alinobogan (which also produced German ellenbogen, Dutch elleboog, and Danish albue). This was a compound formed from *alinā ‘forearm’ and *bogan (source of English bow). However, there is a further twist.
For *alinā (source also of English ell [OE], a measure of length equal to that of the forearm) itself goes back ultimately to an Indo-European base *el-, *ele- which itself meant ‘bend’, and produced not just words for ‘forearm’ (such as Latin ulna), but also words for ‘elbow’ (such as Welsh elin). So at this deepest level of all, elbow means tautologically ‘bend bend’. => bow, ell, ulna
"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.