The story
Taken literally, “you can say that again” gives someone permission to repeat what they just said. Nobody means it that way. It is a friendly bit of exaggeration: the remark was so true, it deserves saying twice.
The phrase is American, and word sources place it in the first half of the 1900s. No one has pinned down who said it first. It belongs to a family of breezy replies, like “You said it!”, that let a listener agree with a grin.
We can watch the change happen. On December 28, 1943, a columnist in the Greensboro Daily News in North Carolina jotted down his “Thoughts while shaving.” The first one read: “The threadbare declaration, ‘You said it!’ seems to have been supplanted by ‘You can say that again!’”
So by the war years the newer saying had pushed out the older one, and it has stayed in the American kitchen-table vocabulary ever since.
