By The By Meaning

(idiomatic) used to introduce a new topic; incidentally

Example: 1990, Susan Sherman, The color of the heart: writing from struggle & change, 1959-1990:
  These sudden rains. Not tropical - with lightning, thunder, great release after hours of tension. But more nonchalant, more "by-the-by." As if the clouds, rushing to get somewhere else, were to drop some rain in passing.