The story
The second "by" is a noun meaning "a secondary issue". Phrase used since the 17th century.
Etymology from Wiktionary, CC BY-SA 4.0.
by the by
Today it means: Used to introduce a new topic; incidentally, by the way. Full definition →
Today it means: Used to introduce a new topic; incidentally, by the way. Full definition →
The second "by" is a noun meaning "a secondary issue". Phrase used since the 17th century.
Etymology from Wiktionary, CC BY-SA 4.0.