Green’s Dictionary of Slang

(US campus use) the grade D.

1967–8		Baker et al. CUSS.
1979		(con. 1964–8) oral testimony in Lighter HDAS I 368/1: When I was in college an A was an ace, a B was a bang, a C was a cat or a hook, a D was a dog, an F was a frog. B+ was a bang and a half.
1980		Eble Campus Sl. Oct. 2: Dog [...] the grade D.
1996		Eble Sl. and Sociability 37: Ace ‘the grade A’, Dog ‘the grade D’, and Zoo ‘the lowest grade possible, Z’ are straightforward expansions.