“I’m down,” the Beatles sang. “I’m really down.” They were not singing about their agreement with where to go after the recording session. Down, in post-beat generation sixties parlance meant, depressed. This derived from the meaning of the word as going in a lower direction and spawned other depressing terms such as down in the mouth, down at the heels and down and out. Then the hippies turned any experience that might cause one to feel down into a downer. A downer could be a bad LSD trip or the rent being due. Years later Saturday Night Live introduced a character called Debbie Downer (no relation), a glass half-empty kind of gal.
Now down’s fortunes are looking up. In the 21st century the word has taken on yet a new meaning. We say, “I’m down,” when we mean, “Yes, I will do that” or “I’m down with that” to indicate “Yes, I agree.”
That’s the magical thing about language. It is always changing. And I am so down with that.