What is "by the by"

Bye \Bye\ (b[imac]), n.

1. A thing not directly aimed at; something which is a secondary object of regard; an object by the way, etc.; as in on or upon the bye, i. e., in passing; indirectly; by implication. [Obs. except in the phrase by the bye.]

   The Synod of Dort condemneth upon the bye even the discipline of the Church of England.
   --Fuller.

2. (Cricket) A run made upon a missed ball; as, to steal a bye.
   --T. Hughes.

3. In various sports in which the contestants are drawn in pairs, the position or turn of one left with no opponent in consequence of an odd number being engaged; as, to draw a bye in a round of a tennis tournament.

4. (Golf) The hole or holes of a stipulated course remaining unplayed at the end of a match.

   By the bye, in passing; by way of digression; apropos to the matter in hand. [Written also by the by.]

prep.phr. (context idiomatic English) used to introduce a new topic; incidentally