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3.
A snooty way of saying "by the way" that should be used with extreme caution.
Man: "What a lovely hawk, by the by, might that be a Ferruginous Hawk? Oh how lovely. Darling, can you believe it?"

Woman: "Dear, you sound so silly when you go off about hawks like that."
by Buster McGooo August 10, 2009
 
1.
Another way of saying "by the way".
By the by, here's the money I own you.
by Kathryn Arwen September 12, 2003
 
2.
one says this when they are about to mention something related to the current subject. Used humorously in Robin Hood, Men In Tights.
"By the by, do you know praying mantis?"
"You're lookin' at him."
by SatiliteStar March 26, 2004