That's because that _is_ the word (大腿) for thigh — native speakers don't perceive it as "big leg", which would be 大的腿 instead.

This is exactly analogous to "little finger" in English.

Except we call that one Pinky

Americans do. It’s not common in the UK.

In Dutch, as an example we have a word called zoetwater - which if you translate it literally, is "sweet water". But it is the word for 'freshwater' in English [as opposed to saltwater], not some kind of sweet water. Even though zoetwater could be literally translated, no one really knows it means freshwater in English. There is advertisement for 'fris water' or 'vers water', which means fresh water correctly.

"fris" is closer to "chilly", tho.

Yea, that's true - in the sense that it's "chilled", or "cool" (i.e. a frisdrank) but not in the sense that it's cold (koud) or chilly outside.

Well, you can say "het is fris buiten", meaning "it's chilly outside".

Same in Romance languages: it acqua dolce vs acqua salata, es agua dulce/salada, pt agua doce/salada (all meaning sweet/salt water).

Turns out that we aren't the only ones that call freshwater "sweet water".

Fresh, non-salt water used to be called sweet in English too, but that term has fallen out of use for some reason.

TIL. Thanks, I wasn't aware of that.

Fun fact: If you follow an extreme lowcarb diet (e.g., Atkinson, where you aim at getting 25g of carbs or less daily) for a long time, some things that didn't have a sweet taste before get a sweet tinge. One of them is water. I remember thinking “Oh, that's why it's called ‘zoetwater’ in Dutch!”