He went to his room to buff the muffin.
buff the muffin
idiomTo masturbate.
Origin
disputed originFrom buff (“to rub”) + the + muffin (“vagina”), chosen for the rhyme in the participle ("buffin' the muffin").
The teenager locked the door to buff the muffin in private.
Finding himself alone for the afternoon, he decided to buff the muffin while the house remained quiet.