From the Super Mario enemy of the same name.
2. A Thwomp is one of the classic Mario Bros, usually found in the Bowser Castles. They come in various different sizes, and are enemies that cannot be defeated at all with neither a jump, Koopa shell, or fireball. If Mario comes near, then it comes crashing down on the designated spot. The best way Mario can get past it is to run/dodge while the angry, strained-looking Thwomp is in the process of moving back up to it's original spot.
2. I had just barely managed to dodge that Thwomp before it came crashing down again, but then, the time to complete that level had just run out.