Please be advised: this entry is incomplete.
Origin unknown.
There are a couple of variations on this. In one, Y is really the short form of X, e.g., in “when I say ‘rare’ what I really mean is that it was thought to be extinct until 1994 when a hiker found a grove of about 100 of the plants in the middle of a one million acre wilderness area outside Sydney, Australia.” (link) In the other, Y is the opposite of X, or they contrast in meaning in some way. E.g., “when I say ‘today’, I mean ‘tomorrow’.” (link)
