What does the word pants mean in the UK?

In the UK, pants are undergarments or underwear. If you're from the U.S. or Canada, pants are the clothes you wear over your underwear, pulling them on one long leg at a time. In most English-speaking countries, the garments you wear under your clothing are known as undies, underpants, or panties, but in the UK, they're pants. If you ask your British friend to show you their new pants, they may look shocked — but anyone from North America will know you mean jeans or trousers. Pants was a 19th-century shortened version of pantaloons, named after a character in Italian comedies of the 1500s who wore tight trousers.

Source: Vocabulary