Shake it three times

To turn yourself on while your urinating - (for males).

Going for a slash
Make sure you "shake it three times" dude.

If you shake it more than three times you’re playing with it

An expression used before or after one takes a piss, meaning that if it takes more than three shakes to get rid of the excess pee it can be considered masturbation.

Tom: Man, I sure have to take a piss

Larry: Well be quick about it, if you shake it more than three times you’re playing with it!