c.1200, "at ease," 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).
Sense of "not difficult to deal with" is mid-14c.; of conditions, "comfortable," late 14c. The concept of "not difficult" was expressed in Old English and early Middle English by eaþe (adv.), ieþe (adj.), apparently common West Germanic, but of disputed origin. Easy Street first printed 1901 in "Peck's Red-Headed Boy." Easy money attested by 1896; to take it easy "relax" is from 1867; easy does it recorded by 1891. Easy rider (1912) was U.S. black slang for "sexually satisfying lover." The easy listening radio format is from 1965, 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.