He went to his room to bash the bishop.
bash the bishop
idiomTo masturbate by stimulating one's penis.
Origin
first recorded 20th c. disputed originUncertain or disputed. Goldman (1950) suggests that beat the bishop comes from a resemblance of the penis to a bishop’s miter; Partridge (7th ed., 1970) likewise derives bash the bishop from a resemblance to the Staunton bishop chess piece. Other sources, such as Green, suggest that assonance of bishop with bash, beat, bop etc. may have given rise to the phrase as a type of word play.
The teenager was embarrassed when his mother walked in while he was bashing the bishop.
Finding himself alone in the house for the afternoon, he retreated to the privacy of his bedroom to bash the bishop.