Depending on the intended meaning:
* Skirt: If you're contrasting the style of clothing, "skirt" is the most obvious antonym.
* Shorts: If you're focusing on the length of the garment, "shorts" could be an antonym.
* Dress: This is a more general antonym for trousers, encompassing any type of garment that covers the legs.
* Nothing: If the context is about whether someone is wearing pants, "nothing" could be considered an antonym.
It's important to consider the specific context to determine the best antonym for "trousers".