quit; drop out; give up; throw in; throw in the towel; drop by the wayside; fall by the wayside; chuck up the sponge.

What does by the wayside mean?

by the wayside. : out of consideration : into a condition of neglect or disuse —usually used with fall.

How do you say by the way in professional way?

The phrase by the way is not especially informal, and you may freely use it in formal situations. However, if you wish to use a variant which is more formal, then you could use a substitute such as: Speaking of which, This brings to mind.