"I am bored to death."
Native speaker here, I'm testing the Chinese course for a friend who wants to use it to learn Chinese. I was definitely surprised by this translation (also just by this sentence generally, what a weird thing to learn.) My answer was 我无聊死了, which is what people usually say. 闷 is sometimes used for the same meaning, but the word is more often used to express feeling depressed. It's also used to describe when air is stuffy and one can't breath.