Green’s Dictionary of Slang

(US campus) a mean, ugly person.

1984		Eble Campus Sl. Mar. 1: brutus – very mean, ugly person.
1996		Eble Sl. and Sociability 46: Brutus is ‘a mean, ugly person’. Clearly it is related to brute ‘animal’ but why is the form brutus rather than bruto or brutage? Possibly the allusion is to the treacherous assassin of Julius Caesar or to the stereotypical use of brutus as the name for a mean, ugly dog.