"to make untidy, put in a state of disorder," 1837, American English, probably a variant of mess in its sense of "to disorder." It was attested earlier (1830) as a noun meaning "disturbance, state of confusion." Related: Mussed; mussing.
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy).
<a href="https://www.etymonline.com/word/mussy">Etymology of mussy by etymonline</a>
Harper, D. (n.d.). Etymology of mussy. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/mussy