Green’s Dictionary of Slang

chach n.

[? Chachi Arcola, character in TV series Happy Days]

(US) a poser.

Urban Dict. 14 July 🌐 A person (usually male) who attempts to exhibit a cool and popular look but generally comes across as either an asshole and/or a total loser.
[US]M. Lacher On the Bro’d 59: Curtis Street [...] is pretty lame; total chaches in graphic tees.