Green’s Dictionary of Slang

(US campus) a general expression of agreement.

1989		P. Munro Sl. U. 138: No doubt! definitely! For sure!
1996		Eble Sl. and Sociability 101: College slang has many expressions that help to keep communication and conversation going. [...] Some express agreement, for example [...] no doubt, word, and word up.
2002		‘Touré’ Portable Promised Land (ms.) 157: We Words (My Favorite Things) [...] Ai-ight. Right on. No doubt. Step off! Stay up.
2005		B. Coleman Rakim Told Me 161: ‘Fly Ty [. . .] asked me to write some stuff for Shante, and I had a lot of respect for her, so I was like, “No doubt!”’.