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1.
The sound of air kissing

Also used to describe a chic restaurant or bar frequented by "beautiful people" (esp. media/fashion/art types), when it is often shortened to a single "mwah"
"I went to the Ivy last night"

"How was it?"

"Totally mwah mwah!"
by Guru Dave September 24, 2012
 
2.
Kiss kiss
used either as an oral greeting or in writing (chat, email, letter)
It is usually associated with chick type of thing but not in all of the cases.
Think Alicia Sylverstone in CLUELESS, WHITE CHICKS, LEGALLY BLONDE
MWAH MWAH! babes see you on Friday, Love You!
by Maria Roz July 31, 2006
 
3.
kiss kiss, but it is verbalized in place of the makeup-smearing facial contact; common in major cities, bar scene people, flamers.
"Mwah, thanks for the free drink dahling"
by jeff at cowtowne January 19, 2004