Masturbating, jerk off, choke the chicken, spank the monkey, beat the meat, whacking the pug, etc. etc.
Manually stimulating your penis with your hand to achieve orgasm.
When you shit yourself with such force it goes through your underwear. The term is from when Bowser ass stomps like in Mario 3 when you defeat him by making him ass stomp through the blocks.
I sneezed so hard I let loose a trouser bowser. I tried to to lift way too much and a trouser bowser was released.