Biffing the cheddar is an ambiguous way of being coy about an action you will or have performed. These acts may be innocent, but are often conveyed with a sexual undertone.
Bro, I spent all night biffing the cheddar to Christi Boner.
Yo Alex! Hurry up in the shitter! What are you- biffing the cheddar in there!?
In the act of sexual intercourse with the anus, or sodomy, the partner ejaculates into the anus and the receiving partner saves the semen in their anus until their next bowel movement. Effectively making a slush puppy in the toilet.
Yo did someone leave a slush puppy in the toilet this morning?!