easyyoudaoicibaDictYouDict[easy 词源字典]
easy: [12] Easy comes via Anglo-Norman aise from Old French aisie, the past participle of aisier ‘put at ease’, which in turn was a derivative of aise. This noun (source of English ease [13]) originally meant ‘convenience’ rather than ‘comfort’. It came from *adjaces, the Vulgar Latin descendant of Latin adjacēns ‘nearby’ (source of English adjacent and related to adjective), which was the present participle of the verb adjacēre ‘lie near’.

The progression of senses is thus ‘nearby’, ‘handy’, ‘convenient’, and eventually ‘comfortable’. The subsequent development to ‘not difficult’, which took place in the 14th century, is purely English, although Breton took the parallel step of borrowing French aise, as aes, to mean ‘not difficult’.
=> adjacent, adjective[easy etymology, easy origin, 英语词源]
easy (adj.)youdaoicibaDictYouDict
c. 1200, "at ease, having ease, free from bodily discomfort and anxiety," from Old French aisie "comfortable, at ease, rich, well-off" (Modern French aisé), past participle of aisier "to put at ease," from aise (see ease (n.)). Sense of "not difficult, requiring no great labor or effort" is from late 13c.; of conditions, "offering comfort, pleasant," early 14c. Of persons, "lenient, kind, calm, gentle," late 14c. Meaning "readily yielding, not difficult of persuasion" is from 1610s. The concept of "not difficult" was expressed in Old English and early Middle English by eaþe (adv.), ieþe (adj.), apparently common West Germanic (compare German öde "empty, desolate," but of disputed origin.

Easy Street is from 1890. Easy money attested by 1889; to take it easy "relax" is from 1804 (be easy in same sense recorded from 1746); easy does it recorded by 1835. Easy rider (1912) was U.S. black slang for "sexually satisfying lover." The easy listening radio format is from 1961, defined by William Safire (in 1986) as, "the music of the 60's played in the 80's with the style of the 40's." Related: Easier; easiest.