Green’s Dictionary of Slang

(US campus) a white person; also attrib.

1965		H. Rhodes Chosen Few (1966) 30: ‘These soda biscuit farmers don’t like nothin’ black.’ [...] ‘What’s a soda biscuit farmer?’ he asked. ‘A cracker,’ answered Blue.
1977		H. Crews Feast of Snakes 68: Soda crackers sho am crazy bout snakes, ain’t they?
1996		Eble Sl. and Sociability 50: Derogatory terms for outsiders are common. For instance [...] a soda cracker, a ‘white person’.