by the by

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by the by

Incidentally; on a side note; by the way. An interjection meant to casually introduce or emphasize additional information in the conversation. Sometimes, though less commonly, written as "by the bye." By the by, I remembered where I left my keys, in case you were wondering. Beckett was an amazing playwright. There's a lecture on his work tomorrow, by the by.

by the bye

Also, by the by. Incidentally, in passing, as in By the bye, my wife is coming too, or Exactly where do you live, by the by? The bye or second by in this term originally meant "a side path," whence the current sense of "off the track" or "of secondary importance." [Early 1500s] Also see by the way.
See also: bye

by the by (or bye)

incidentally; parenthetically.