It’s from libra pondo.

The word pound comes from the Latin word pendere, meaning “to weigh”. The abbreviation lb comes from the Latin word libra, meaning “scales, balances”, which also described a Roman unit similar to the pound.

On a semi-related funny story, I always made fun of my first grade teacher because she couldn’t comprehend how plural lb. and lb’s. worked

I was gonna say the same reason ounce is abbreviated oz, but Nimis’ and Blue Freedom’s answers make much more sense.

Oz. is from onza!

the fatter you get the more you look like a “b” then a “B”