Dreamt is widely cited as the only common English word that ends in -mt. That makes it memorable, because English words rarely finish with that letter pattern.
The claim usually includes the word common for a reason. English has a few very rare, technical, archaic, or proper name exceptions, but in everyday vocabulary, dreamt is the standout example people mean.
There is also an important usage note: dreamed and dreamt are both correct past tense forms of dream. Dreamed is more common, especially in American English, while dreamt appears more often in British English and in more literary styles.
- Example: I dreamed about missing the train.
- Example: She dreamt of living by the sea.
So the fun fact is not that dreamt is the only possible word of any kind ending in -mt, but that it is the only one most speakers are likely to know or use. That mix of rarity and familiarity is what makes it such a popular grammar fact.