1 definition by Wordslinger for Hire
Like "before I/we forget," "by the bye" means the speaker (or writer) is going to change the subject. It comes from "by" (meaning "near") "the bye" (the closing, where we say "goodbye.") So it means, "before we say bye, let me also tell you...."
"... and that's all I heard about Brenda and Eddie. By the bye, have you heard from Brenda's friend Billy?"
by Wordslinger for Hire November 27, 2013