A sexual act, usually performed by male virgins, whereby they incorrectly slide their penis between the woman's thigh and bedsheet, and think they are having vaginal intercourse
"This blokethought he was inside me, but he was thigh-sheeting. I let him have it"
When you share a bed with someone you don't want to have sex with, so you lay on top of the top sheet to prevent contact. When alcohol is involved usually works.
If you don't want to have intimate situations with drunk bed mate. Sheeting usually works.
Fall asleep on a different layer, usually they will give up!