Mariah Carey may have just fired her first shot at Nick Cannon ... singing, "I know you cheated, motherf***er."
Mariah was in Tokyo ... singing Billie Holiday's "Don't Explain," a song about a cheating lover.
Listen closely ... right around 7 seconds in, it sounds like she changes the lyrics from "And I know you cheat/Right or wrong, don't matter" to "I know you cheated motherf***er."
So whaddya think?
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