c. 1200, "morally evil; offensive, objectionable" (other 13c. senses were "malevolent, hurtful, unfortunate, difficult"), from Old Norse illr "evil, bad; hard, difficult; mean, stingy," a word of unknown origin. Not considered to be related to evil. From mid-14c. as "marked by evil intentions; harmful, pernicious." Sense of "sick, unhealthy, diseased, unwell" is first recorded mid-15c., probably from a use similar to that in the Old Norse idiom "it is bad to me." Slang inverted sense of "very good, cool" is 1980s.
c. 1200, "wickedly; with hostility," from ill (adj.). Meaning "not well, poorly" also is from c. 1200. It generally has not shifted to the realm of physical sickness, as the adjective has done. Ill-fated recorded from 1710; ill-informed from 1824; ill-tempered from c. 1600; ill-starred from c. 1600. Generally contrasted with well, hence the useful, but now obsolete or obscure illcome (1570s), illfare (c. 1300), and illth.