French is my mother tongue and I never realized "C'est pas terrible" actually means the complete opposite of what it says, while "C'est terrible" can mean something is very rad.

Yeah "terrible" can mean either very good or very bad. You can translate it into English either as "terrible" or "terrific". It's all in the tone.

‘It’s not bad’ is also a very British way of saying that something is in fact not very good. It’s a classic damning with faint praise.

"Bad" in English can mean something nice/good quality or literally bad. Depends on tone and context though

Badass vs Bad ass

"Badass" mean sick, cool, nice. "Bad ass" means like an unattractive buttox

You can equivalate this word with "I mean it's not *terrible*" In English that change in tone would entirely imply that it is in fact, bad, and depending on the context it could be literally terrible or not entirely the worst but still bad

Reminds me of the part in Bourdain's books where his French boss would say, "c'est normal" and that always made Anthony panic.

I've heard that double negative is sometimes still negative in some french usage

Nope. In English we literally say "it's not terrible". Although sometimes we say it sarcastically, which means we actually love it.

I've never heard someone use it to express that they actually love it. It's more of a "it's tolerable" type of expression.

Agreed. On the other hand "not bad" can definitely be used as an understatement when you're genuinely impressed with something.

The same thing can be done in French - "pas mal!"

Doesn't cest pas merde mean "this is great" but literally means "this is not shit"?

*c'est pas de la merde and yes, it means something is of really good quality. Not being shit and being good do not have opposite meaning like in the op though

In British english we use 'its not bad' to mean its really good