Best definition
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’