What does Boxing Day have to do with boxing?
early 15c., from wild (adj.) + cat (n.). Meaning "savage woman" is recorded from 1570s; sense of "one who forms rash projects" is attested from 1812. The adjective in the financial speculative sense is first recorded 1838, American English.
Madly exuberant; untamed; crazy, wild: Shepard tops himself as a wild-ass country boy/ and for goddamned sure a wild-assed warrior/ Ijah was a kind of wild-ass type
[1960s+; an example of the use of ass, ''the whole person'']