1. Too simple to bother detailing.
2. Not worth the speaker's time.
3. Complex, but solvable by methods so well known that anyone not utterly cretinous
would have thought of them already.
4. Any problem one has already solved (some claim that hackish `trivial' usually evaluates to `I've seen it before'). Hackers' notions of triviality may be quite at variance with those of non-hackers. See nontrivial
The physicist Richard Feynman, who had the hacker nature to an amazing degree (see his essay "Los Alamos From Below" in "Surely You're Joking, Mr. Feynman!"), defined `trivial theorem' as "one that has already been proved".