beating off
1) Slang term for male masturbation.

2) Grasping a penis, then stroking vigorously in and up and down motion in order to produce an orgasm. Usually accompanied by ejaculation of semen.

See also: jerking off; jerkin the gherkin; jacking off; wanking; whacking off; beating your meat; chocking your chicken; spanking the monkey; slapping the salami; tickling the pickle
When your little brother is taking three showers a day, each for 45 minutes...he’s probably beating off!