To stiffen someone means to give someone a right good beating so they are stiff in the
morning. If you stiffen someone they won't
know what's hit them.
Said to have originated in Glasgow, Scotland.
Example 1:
Jock: Here mate did you see stiffen
Bobby last
night?
Div: Oh aye, he will be stoatin about this morning!
Example 2:
Jock: Shut it or all stiffen you!
Davy: Awright the come ahead ya
fud!