In British English, the term pants refers to men's underpants. It is a catch-all term that can denote anything from thongs to boxer shorts, but all have the common feature that they are worn under trousers.
The word "pants" has acquired humorous connotations in the last few decades, largely among younger people. Many stand-up comedians have used the word as a lightly pejorative term. There is a tendency to use it as an adjective. This usage came to wider attention when it came to light that the then Prime Minister John Major tucked his shirt into his underpants.