twiddle my diddle stick
To twiddle your diddle stick means to masturbate, but your penis is very thin, and almost like a stick, but you don't have any lubrication, so you have to smack it back and forth with one hand, as if you're twiddling it.
Person 1: I'm going to twiddle my diddle stick tonight!!
Person 2: ...Dude this is a playground...
Person 2: ...Dude this is a playground...