The typical Scottish lad lifestyle of getting into other girls, where;
1. Graft her on Snapchat, Messenger, etc.
2. Plan or arrange to meet at a Gaff (house party)
3. Shag (have sex) with the girl in a bedroom of the same party (or similar)
It is then customary to follow the Pump and Dump scheme to then repeat the process with a different girl.
"John you planning on getting stuck into Susan tonight?"
"Aye, doing the old Graft Gaff Shag with her"
What you say to someone with whom you have had one or more previous sexual encounters. Primarily used to
1. Announce sexual prowess and coolness to the general public.
2. Display lack of shame.
3. Humiliate the person to whom the comment is directed, automatically making you the winner and czar of all things excellent.
Ethan: Yo Melissa! Long time no shag, eh?
Melissa: Go fuck yourself Ethan.