Like "before I/we forget," "by the bye" means the speaker (or writer) is going to change the subject. It comes from "by" (meaning "near") "the bye" (the closing, where we say "goodbye.") So it means, "before we say bye, let me also tell you...."

This is a play on the phrase by-and-by which means: some future time or occasion.

By changing the spelling it implies that we are saying goodbye until some future time or occasion; til later on.

This is an especially useful phrase when you want to say goodbye to someone that has passed away.

what i would very much like to say to George H. W. Bush

When something you say can go both ways, either good or bad, like a bisexual.

Used as a responding word, like "lol, "lmao" and "omg I can't even". Figuratively meaning that your leaving, like you can't deal with what has been said.

Bye is a way to say get the fuck out

A simple way of concluding a conversation.