Short for "shut your mouth or feel like a trout" which is a derivative of "You had better shut up unless you want a pocket full of water"
Hey P why don't you hurry up?

P: "mouth or trout"
by cb February 20, 2003
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You should've seen the red mouth trout on that girl, had me scared go diving!
by BeatinCheeks April 12, 2019
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