c.1200, "morally evil" (other 13c. senses were "malevolent, hurtful, unfortunate, difficult"), from Old Norse illr "ill, bad," of unknown origin. Not considered to be 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.

early 13c., "to do evil to," from ill, adj.. Meaing "to speak disparagingly" is from 1520s. Related: Illed; illing.

c.1200, "wickedly; with hostility;" see ill, adj.. Meaning "not well, poorly" is from c.1300. It generally has not shifted to the realm of physical sickess, 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.