To be being beaten heavily in sport or business. If you are the one that is being beaten then you are being "dicked on". If you are beating someone else then you are "dicking on" them.
To make a shot during basket ball which in this case would be a slam dunk before the other team can react. Also what Sans says at the end of Genocide in Undertale.
Timmy went for the basket and afterward shouted "Get Dunked On!"
Sans tricked you and said "Get Dunked On!" Not again you thought.