Answer: MODESTY

Modesty refers to the quality or virtue of being humble, unassuming, and not excessively proud or showy. In the context of the clue, "The virtue of the lukewarm" according to Sartre, it suggests that modesty can be seen as a middle ground between being overly confident or self-assured and being overly self-deprecating or lacking confidence. It implies finding a balance in one's behavior and demeanor.

Note: The provided meaning of the answer is for the latest New York Times crossword puzzle clue: "The virtue of the lukewarm," per Sartre. Explanations are generated by AI and may be inaccurate.