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 by, I'll be home late tonight. The printer jammed, so that set us back with finalizing our report.

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.”