A person who cannot let anything go by. They always have to answer back in a condesending way. These are usually people who are higher in the academic world than their peers and they use their knowledge in a mean way.
"He had to say that just to be a clever trousers. Who cares if pigsdon't really sweat that much?"
It's a word you use in conversation about anything. For example, if you are talking about aircraft carriers, you can say, "As recent as yesterday I owned a pair of trousers that looked exactly like an aircraft carrier."