Green’s Dictionary of Slang

(drugs) LSD.

1969		R.R. Lingeman Drugs from A to Z (1970) 71: crackers [when Boston police became aware that LSD was being put on sugar cubes, users switched to putting it on animal crackers] lsd-25.
1982		AS LVII:4 289: A sampling of current names for varieties of LSD would include [...] crackers.
2001		ONDCP Street Terms 7: Crackers — LSD.