Green’s Dictionary of Slang

(US campus) a mild excl., synon. with SE wow!, gee! excl., shit! excl. (2) etc. and generally implying agreement or approval.

1989		P. Munro Sl. U. 74: Dude! Hey! Wow! Gee! Shit!
1992		D. Burke Street Talk 2 17: Dude! Look at that!
1994		Eble Campus Sl. Apr. 3: dude – response covering a range of meanings from agreement to incredulity.
1997		Eble Campus Sl. Apr. 3: dude – [...] affirmative response.