MERRIAM-WEBSTER
Was this all a joke?
You could be excused for thinking that much of English pronunciation was invented by a trickster god, one with a particularly cruel streak. After all, how else could we have come to a place where through doesn’t rhyme with though, enough doesn’t rhyme with lough, and cough doesn’t rhyme with hiccough? We’re happy to tell you that there was no trickster god involved: there are reasons for why things are the way they are. Read on, and we’ll explain one of the great mysteries of our language: why so many of the letters seem to be just sitting around doing no work.
mnemonic
Some letters are silent in English because they are part of sound combinations that are so uncommon that English speakers ultimately resist pronouncing them. Our language is a glutton, and it has taken words from an enormous number of other languages. Since we have words borrowed from languages that have different sound patterns, this results in English speakers pronouncing the words differently than in their languages of origin.
That’s why the m is silent in mnemonic, a word meaning “assisting memory” or “relating to memory.” Mnemonic came to English from Greek through Latin during the 1600s, when many words of Classical origin were introduced by scholarly writers.
It is documented that the m was pronounced before the n as recently as the late 1800s, and has since dropped away.
There are very few words in English that begin with ¬mn, and most of them are rare words that share the ultimate Greek root of the word meaning “to remember,” including mneme (pronounced /NEE-mee/), mnestic , mnemotechnical, and the name of the Greek goddess of memory, Mnemosyne.
Merriam-Webster for more