The word trousers typically refers to the long garment worn on the legs and which covers the whole leg.
There are no categorical antonyms for this word.
However, one could loosely use garments worn on the upper as antonyms, e.g., shirt, blouse, t-shirt, etc. Alternatively, one could loosely use shorts - a garment which partially covers the legs - as an antonym.
Noun
A garment that extends from the shoulders to the waist or hips
