1. Fabric:
* For the fabric itself, "silk" or "linen" would be good antonyms. These represent finer, smoother, and potentially more luxurious fabrics.
* "Cotton" could also be considered an antonym if you're talking about the specific kind of flannel made from cotton.
2. Clothing:
* "Formal wear" or "suit" would be antonyms for a flannel shirt, as they represent a more dressed-up style.
* "T-shirt" could also be an antonym if you're looking for something more casual and less bulky.
3. Slang:
* "Polished" or "sophisticated" could be antonyms for "flannel" when it's used slangily to mean something unfashionable or unpolished.
Ultimately, the best antonym for "flannel" depends on the context.