When you tittyfuck a girl's ass. Your penis will move up and down in between the cheeks without entering the anus. Only usable on a girl with a tight ass.
Yo man I was trenching that bitch so hard last night
Trenching: The act of having sex without penetration by simply putting your penis between the woman's legs in her thigh gap and thrusting, or by rubbing the penis between the labia ie. In the trenches.
Dude she was a prude and wouldn't let me bang her, but at least I got of while trenching her.
Trenching v. -To dance, jump, scream and sing, whilst pushing, shoving and potentially biting other audience members in the 'trench pit' of a concert. This is formally known as "moshing".