Answer: JUSTYOUWAIT

The phrase "just you wait" typically conveys a warning or a threat. It suggests that the speaker is confident in their abilities or actions and implies that they will prove themselves in the future. In this context, the additional clue "Oh, I'm not messing around!" emphasizes the speaker's seriousness and determination.

Note: The provided meaning of the answer is for the latest New York Times crossword puzzle clue: "Oh, I'm not messing around!". Explanations are generated by AI and may be inaccurate.