The only common English word traditionally cited as ending with the letters “mt” is “dreamt.” It is a past-tense and past-participle form of “dream,” as in “She dreamt about flying” or “They had dreamt of moving abroad.”
“Dreamt” is an irregular form because it does not use the regular -ed ending. The alternative “dreamed” is also completely correct. Dreamt is especially familiar in British, Irish, Australian, and other varieties of English, while dreamed is often more frequent in American English.
The claim needs the word “common.” Related forms such as “undreamt,” “daydreamt,” and “redreamt” can also end in “mt,” but they contain dreamt as part of a compound or prefixed word and are much less frequent. Words such as “attempt” and “tempt” end in “pt,” not “mt,” which makes them tempting but incorrect choices. The spelling reflects the unusual consonant sequence rather than a special pronunciation rule.