adv.(do something) in addition to what one is already doing, without much extra effort
When you go to the library, please return this book for me.
I drop in to see you guys on my way home from work.
Please take this study material with you to the workshop.
I mention this point now in passing and shall refer to it again.
I take this opportunity to report to you about a matter.
drop in at a shop
by the way
mention in passing
drop a few suggestions
My mother dropped round that day.
Friends are only incidental to the main purpose of our visit.
visit friends in passing
happen to resolve another issue
put it off till later
deliver a letter to him in passing
When you go to buy that book get me a copy as well.
He raised the matter in passing.
say in passing
It was just said in passing.
At this juncture it would be convenient for me to mention the following particulars.
Go and see what's the matter, and while you're about it you can fetch me my sweater.
I'll look in at the store.