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Well it is notch first of all and secondly it means calm down!
You tell him How small that is. Take him down a notch.
Take it to a qualified gunsmith.
slang: make them depressed or take them down a peg/notch
literal: release air from the tires of their auto or bike through the valve
Then take it down a notch... Don't go so fast, and let her know you are sorry for making her feel pressured.