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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!
by Jock McTavish March 22, 2013