Green’s Dictionary of Slang

(US campus/teen) an expression of approval or surprise and disbelief.

1987		Eble Campus Sl. Oct. 2: daag: exclamation of disbelief: ‘I broke up with my boyfriend last night.’ ‘Daag!’.
2002		updateghost ‘Rev. of 8 Mile’ at Epinions.com 10 Nov. 🌐 We had sneak into this sh*t by buying a ticket to see that f-ckin’ stupid Ring movie yo, and den we had to talk to dis guy who mah girl knew who worked at the theater to get us in past security and sh*t, but DAWG, it was AWESOME.