Why do we say "If you know what I mean"?

Meaning:

Usually used ironically following an unclear statement

Background:

According to The Random House Dictionary of America's Popular Proverbs and Sayings this catch phrase originated in the twentieth century. One of the earliest recorded usages was by F Scott Fitzgerald in The Great Gatsby (1925).

"I like your dress," remarked Mrs. McKee, "I think it's adorable".
"..... It's just a crazy old thing," she said. "I just slip it on sometimes when I don't care what I look like."
"But it looks wonderful on you, if you know what I mean," pursued Mrs. McKee."

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