MWAH

You've seen it on television, or in the street, hundreds of times, thousands of times. Two people come towards each other, they obviously know each other very well, and they start to kiss each other - but it's not a full frontal kiss. No, what happens, one person puts the cheek against the other person's cheek and they have what is often called an 'air' kiss.

They make a kissing noise, which shows that they're coming together, as great intimates, but it's not a real kiss at all. And many people then give this air kiss a noise, a word, and it's usually 'mwah', 'mwah' - something like that.

Now, how do you write it? Well nobody knows quite how to write it, but it's really m-w-a-h. I saw it written in about the mid-nineties for the first time. And, there's a plural too: "there's lots of mwahs about these days"

http://www.bbc.co.uk/worldservice/learningenglish/radio/specials/1728_uptodate/page11.shtml