Short for, "tell me when to stop". The grammatically improper, but obedient reply is, when. Some people who use this phrase are so pedantic about receiving an obedient reply that they will not stop if the reply is other than, "when", because they think it's funny.
Say when.

OK... That's enough.

I said that's enough!

Will you stop already?!?

When you say "when".

When, already! You've ruined my drink!!!
by Downstrike October 30, 2005
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What Tom (god) says (Tombstone)

Basically, this is a subtle nod to the famous epic movie of a spaghetti western namedTombstone, when the titular Doc Holiday says "Say when"
Tom: "Hey Dayveeitd."
Davit: "Yeah??????"
Tom: "I got a good one-uh."
Dayveit: "Okay...?"
Tom: "I'M DOC HOLIDAY Say When!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!" ;3
David: *walks awayuhn XD*
by Hilfred Steelhelm May 30, 2020
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