An uncertain term which may refer to beings who were literally half Troll and half Man, or may simply be a simile used to suggest that certain troops of the Haradrim were truly huge and terrifying, while not actually being any more than Men.

The term 'half-trolls' only appears in the single citation given above, and purely on that basis, it appears that Tolkien is using the term more for literary effect than to imply some kind of cross-bred being. However, later in the same chapter there's a direct reference to 'Troll-men', apparently the equivalent to 'half-trolls', which strengthens the possibility that these people actually did carry Troll blood.

Neither half-trolls nor Troll-men are mentioned outside these two rather indefinite references, so ultimately it's impossible to be sure whether there really were literal half-trolls in Middle-earth. It seems at least as likely that the Men of Far Harad were simply so huge and ferocious that they could meaningfully be compared to Trolls.