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ease (n.)[ease 词源字典]: c. 1200, "physical comfort, undisturbed state of the body; tranquility, peace of mind," from Old French aise "comfort, pleasure, well-being; opportunity," which is of unknown origin, despite attempts to link it to various Latin verbs; perhaps Celtic. According to OED, the earliest senses in French appear to be 1. "elbow-room" (from an 11th century Hebrew-French glossary) and 2. "opportunity." This led Sophus Bugge to suggest an origin in Vulgar Latin asa, a shortened form of Latin ansa "handle," which could be used in the figurative sense of "opportunity, occasion," as well as being a possible synonym for "elbow," because Latin ansatus "furnished with handles" also was used to mean "having the arms akimbo." OED editors add, "This is not very satisfactory, but it does not appear that any equally plausible alternative has yet been proposed."

At ease "at rest, at peace, in comfort" is from late 14c.; as a military order (1802) the word denotes "freedom from stiffness or formality."[ease etymology, ease origin, 英语词源]
elbow (n.): "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.