Natt is right. Pants are only trousers in U.S. English. There are many, many other varieties of English. But I will agree that when I say trousers consistently for the long-legged outerwear in the U.S., I frequently get funny looks.
I've been told by a native speaker that it's not polite to use this word in public. People say instead "nguo ya ndani" when talking about underwear in public.