Everyday vocabulary changes over time, as it is supposed to. But slang can change so fast, what you thought you were up on has already changed to something else, as everyone knows but you, like your grandchildren snickering behind your back. That’s why anyone past twenty and who has a life should avoid slang altogether.
I recently heard on Girl Code, “Keep it on the DL.” DL stands for “down low,” meaning quiet, secret—not deep, dark secret, just don’t gratuitously spread it around. We would have said, “Keep it on the QT,” and winked.
But “on the down low” just a year or two ago had a very specific and different meaning. It meant a homosexual relationship kept secret from a heterosexual partner. You could hear something like, “Her boyfriend is living on the down low. Been so for years.”
This is typical. Slang often becomes acceptable as it broadens in use, and its earlier, rougher meaning is forgotten.