on the down-low

A friend of mine, about to relay some clandestine information, said she was going to give me "the down-low."

"I think you mean 'the low-down,'" I said. "The down-low is the secret gay sex one."

To be fair, though, I always used to mix up "down under" (meaning Australia) with "down below" (meaning vagina, if you're an old lady).