Had absolutely no idea, so I cheated and used mighty Google!
It's from Zelda II, the Adventure of Link. Actually, it's a hint in the town of Mido that tells the player how to gain access to a locked building, wherein they can learn the down-strike.
I don't think I found that secret, but then again I did really poorly in Zelda II, so such is to be expected.
EDIT: I won't post one until it's been verified that I am correct.