The sound made (a little like a duck's quack) when somebody does/says something stupid or is on the receiving end of some bad luck or event. Occasionally, the word can be repeated for extra effect.
Lauren: 'My grandma just died'
Laura: 'I just feel like my career is going nowhere'
Lauren: 'Mwak mwak'
Lloyd: 'I didn't know you could put foil in the fridge?'
Jess: 'Mwak mwak'