The word is ultimately dismissive in that it is usually to be found as a closing salutation and it has a subtle undertone of a dismissive frustration that was never happening and was never meant to be being replaced with a more sustainable ambivanence, and therefore, happiness.
A sound said or written in the start of an sentence(most times anyway), to express something related to 'negative' things, or as a no.
Also used as an evil laugh, short for mwahaha.
#1
Person: Hi, wanna go buy some cake?
Other person: Mwah, i'm broke.
#2
Person: *falls down the stairs*
Other person: Mwah! xD