A lout is a clumsy, awkward oaf. If you want to insult the person who just stepped on your toes and belched, say, "Watch where you're going, you lout!"
Lout originally meant a bumbling, awkward fellow, both annoying and pitiful. But if you call someone a lout today, you're also implying that he's not only stupid and clumsy, but that he's no good. Lout is almost never used for women, because the word has an implication of being bad to women — not abusive so much as selfish. If everyone thinks your boyfriend is a lowdown, disgusting lout, you might want to try dating someone else.