a finger in the bum - When you put your finger up your butt while you're wanking a

When you put your finger up your butt while you're wanking and you've got soap all over it because you're too cheap to buy lube.

I did it in the shower after I ran out of lube.

My friend did it while he was cleaning the kitchen.

I did it during my math class because I had no lube and no time.