Being called a 'trot' is a derogatory term. I have a personal dislike of that kind of abbreviation anyway, because it dehumanises people.
In any case, Corbyn is not a Trotskyist. I can't be bothered explaining what Trotskyists believe in, but a read of Wiki gives a general idea. There are, at the very most, a couple of thousand Trotskyist activists in the UK.
Just seen your link to Peter Hitchens, dj. Peter Hitchens is opinionated and often arrogant, but he's an astute observer and a good writer. He's right about so-called Trotskyist bogeymen. There are far more neo-fascist, right-wing, racist and even complacent bogeymen.