Seething describes anger you can barely contain. If you are seething and something sets you off, you just might explode into a rage.
Someone who's seething with anger is furious but keeping it a secret. Maybe someone insulted you in front of others and you want to let it go — on the inside, though, you might be seething. Or during the big game, an opponent's harsh words can have you seething, but you want to stay focused on scoring points. Seething sometimes describes something that's boiling over, like a seething pot of soup, and this is also the root of the word.