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 African-American vernacular 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.