Incidentally; used in referencing a parenthetical statement not timely, central, or crucial to the topic at hand; foregone, passed by, something that has already happened.
adv
Irrelevantly, off-topic.
unimportant
adj
petty; not important or noteworthy
How often have the words by the way and unimportant occurred in a corpus of books? (source: Google Ngram Viewer )