Similar to a neck, a Shag is a way to let someone know that they said or did something dumb by using your hand to act like you are pulling the thought out of their head.
Shag means sleep. For instance you would say something like
“I shagged for 8 hours last night!”
And instead of sleep over, you say shagover. For instance you would say
“I had a shagover with my best friend!”
That’s telling someone you had a sleepover!