Short for Trotskyist. Usually but not always an offensive term, used by opponents of Trotskyism of both left and right persuasions (usually as a descriptive noun, "the trots").
In some settings, such as the National Union of Students, the insult is so pervasive that anyone to the left of Hitler is liable to be labelled a "trot" at one point or another, because the NUS right-wing associate any challenge to their rule with a conspiracy allegedly initiated by a long-defunct Trotskyist group called Socialist Organiser.
The term also crops up sometimes in anarchist and ecological attacks on Trotskyists. It is also occasionally used by Trotskyists themselves, apparently as a "reclaimed" term, only heard within the Trotskyist in-group, although this is contentious and some reject the term completely.
Its derogatory implications probably come from its associations with the trots
in its other sense.