"Hep" is a term that was popularized in the 1940s and 1950s, particularly within jazz and African American communities. It is an adjective that refers to being stylish, trendy, or fashionable. However, as stated in the clue, it signifies something that was once considered cool but is no longer in vogue.
Note: The provided meaning of the answer is for the latest New York Times crossword puzzle clue: With it, in old slang. Explanations are generated by AI and may be inaccurate.