early 14c., "unfortunate, disastrous, miserably, wretchedly," probably from mischief + -ous. Sense of "playfully malicious or annoying" is attested by 1670s. "The stressing on the second syllable was common in literature till about 1700; it is now dialectal, vulgar, and jocular" [OED]. Related: Mischievously; mischievousness.
updated on January 27, 2019