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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
by the bye
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
by the by (or bye)
Farlex Partner Idioms Dictionary © Farlex 2017
By the way; incidentally.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
by the by/bye
By the way, incidentally, as in “By the by, I saw your brother there as well.” The second by or bye in this archaic-sounding cliché originally denoted a side path, whence the current sense of “in passing.”
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer