cough cough is used to subtly imply that you know something that someone else doesn't. The person who it is "used on" will then begin to suspect something is suspicious with the phrase "cough cough" and will begin to wonder what it means. Mostly, "cough cough" is pronounced as a word, instead of being actually "coughed out".
Harry: "Hey, I heard Sue went out with Mark"
BoB: "oh yeah, cough cough"
Harry: *wdf, does he know something that I don't? Holy crap he's H@Rd<0R3*
The act of semi-discretely littering in a way that looks as though accidentally dropping the trash while suddenly and deeply coughing.
I was getting tired of holding that empty soda cup so I cough-coughed it out the window.
1. any act involving sexual activity with another of your species
Leon: hmmm i need Jo badly
Al Al: why is that bro?
Leon i need some cough-cough
Al Al: oooooooo hope she gets on soon