This termonology can be used in two scenarios, the first being used as a insult, usually behind someones back, and is a derogative from the 'Wanker' and the term comes from someone mumbling it.
The second scenario can be used as a cry, as appose to a snide comment, shouting it out as someone trips over or makes a blunder
Example 1
"Hey you seen that fat kid trying to score with that new girl?"
"*sniggers* Wangaa"
Example 2:
'After two teenagers see a elderly gentleman fall over one cries out'
"WANGAAAAA!"
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