This is the term used when someone gives you an insult and a compliment in the same sentence. Salty comes from inSULT and mint comes from compliMENT.
"Wow! That's the most interesting thing you've ever said to me"
"You just gave me a salty mint, sir"
-Its a compliment because you like the thing they just said but also a insult because everything they said before is terrible.