I may be the only one, but I love finding out where letters come from. It’s so weird which sounds we pick to have symbols for and which ones we don’t (no “ch” letter, although it’s a common sound, nor “th” anymore), and the fact that sometimes we have more than one letter for a sound (look at the overlap between C and S, C and Q, or J and G). And when you try to figure out the whys of letters, the best explanation you can come up with is “because”. Ah, linguistics!
The history of the letter M is pretty neat as the symbol has remained fairly constant throughout history. Okay, I think it’s neat. The modern Latin version is virtually identical to the Etruscan M, although some versions have an elongated line at the end similar to the Greek lowercase mu (μ), while the capital Μ is, as I’m sure you noticed, no different from what we have. Hundreds of years, thousands of miles, but M is M.
Brief side note: I’ve mentioned this a few time (like, in every letter post) but the Romans who popularized our alphabet took the letters from Etruscan, a now extinct language. The Etruscans in turn copied their alphabet from the visiting Greeks, who were the first in Europe to have an actual alphabet—which is defined as a letter system with both consonants and vowels. And if you go all the way back to ancient Greek, M still doesn’t look all that different. Some dialects have the swoop of the mu more pronounced, but the two peaks remain constant until it evolved to the more modern form.
Now, the Greeks were the first to come up with a traditional alphabet, but they weren’t the first to have one. The Phoenicians had what’s known as an abjad, an alphabet with only consonants, no vowels, and their mem is basically the ancient mu flipped around. The Phoenician alphabet evolved almost four thousand years ago from a Proto-Sinaitic base where the symbols were taken from Ancient Egyptian hieroglyphs. The shape of M starts to make more sense when you realize that the proto-Sinaitic mem came from the Egyptian word for waterand the jagged line symbolizing it was supposed to look like wave crests.
TL;DR: M is wave peaks.