Green’s Dictionary of Slang

dog v.4

[dog n.11 ]

(US campus) to get a grade D in an examination.

[US]Eble Campus Sl. Fall 2: dog – the grade D [...] ‘I’m doggin’ this class right now.’.
[US]Eble Sl. and Sociability 69: As a verb dog specializes in context to mean ‘make the grade D or do poorly’.