“M” is for “Cat”
The Ancient Egyptian word for cat is pronounced mioo with a short i like it, or if you can, a stop like a short hiccup between m and oo. Did you notice how much it sounds like “meow“?
The cat picture is called a determinative. It isn’t a letter, but it helped keep people from getting it confused with other groups of words that might be spelled “mioo“.
Cats were so common that eventually the cat symbol alone became short for mioo.