When someone with a group of friends(typically online) says, "hey I'll be right back." But doesn't return. Later returning as if never leaving at all.
The term is used because the role played in said game(s) is crucial.
"Yo he just foshed us hard."
You totally foshed me last night dude
Dude we need Mikie to get anything done tonight, I hope he doesn't pull a fosh and go to 7/11