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 uncertain origin. According to Watkins is ultimately from Latin adiacens "lying at," present participle of adiacere "lie at, border upon, lie near," from ad "to" (see ad-) + iacere "to lie, rest," literally "to throw" (from PIE root *ye- "to throw, impel"). Recent dictionaries in English and French seem to support this derivation, though older sources found objections to it. Compare adagio.
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."
c. 1300, "to help, assist," from Old French aiser, from aise (see ease (n.)). Meaning "to give ease, mitigate, alleviate, relieve from pain or care" is from mid-14c. Meaning "render less difficult" is from 1630s; the sense of "to relax one's efforts" is from 1863 (with up by 1907, earlier with a more specific sense in sailing). Farmer reports ease in a slang sense of "to content a woman" sexually, with an 1861 date. Related: Eased; easing.