c.1200, "morally evil" (other 13c. senses were "malevolent, hurtful, unfortunate, difficult"), from Old Norse illr "ill, bad," of unknown origin. Not related to evil. Main modern sense of "sick, unhealthy, unwell" is first recorded mid-15c., probably related to Old Norse idiom "it is bad to me." Slang inverted sense of "very good, cool" is 1980s. As a noun, "something evil," from mid-13c.