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".

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.

when your not in agreement with something but the other person things you’re in agreement

The act of semi-discretely littering in a way that looks as though accidentally dropping the trash while suddenly and deeply coughing.

1. any act involving sexual activity with another of your species