When you're trying to show someone online that you are saying something under your breath, use the cough tag.
Joe: Duuuude, I got the awesomest iPod yesterdayy!!!!!
Bob: *cough* gay *cough*
Joe: Huh?
Bob: Huh?
Joe: Did you just use the cough tag on me?
Bob: Uh, no, that wasn't me.
Joe: But you- but- ugh.