The term "most old chestnut," refers to an oft-repeated, hackneyed story or idea. In terms of antonyms, one could look to describe something that is fresh, new, and original. Depending on the context, antonyms might include terms such as innovative, unique, cutting-edge, or groundbreaking. Alternatively, one might seek to emphasize the way in which "most old chestnut" implies a sense of outdatedness or irrelevance by exploring terms such as current, modern, up-to-date, or contemporary. Ultimately, the antonyms for "most old chestnut" are likely to vary widely depending on the specific circumstances of its use, and the connotations and meanings that it holds in a given context.