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1.
The act of knocking someone unconcious in one punch.
Oi, give 'im the one bang!
by Tobedottty January 27, 2009
 
2.
Meaning is in a fight knocking out the opponent with one punch, or hurting the oponent to the level he gives in or walks off (pussy)
"dont chat shit or i'll oneBang you!"
by hype kid April 13, 2010