I was just thinking about this randomly today...
At least in American English, you can use the phrase "up for something" to express that you want to/ are willing to do something.
For example: "I'm up for going to the mall."/ "I'm up for it." (Though, you can't really say simply "I'm up.")
However, you can also say "down (for something)" and it has the same meaning!
For example:"I'm down for going to the movies."/ "I'm down."
It seems a touch slangier than the "up" phrases, but I think it's pretty interesting that the words are opposites and still convey the same meaning.
Is this a more common linguistic occurrence than I think? Any other examples in English or other languages are welcome.