A person who possesses little or no social skills. They are bigoted and prone to aggressive outbursts and tantrums. They are highly critical and gain much joy from pointing out people's weaknesses. They believe they are intelligent, yet due to their stubbornness they fail to see when they have made a mistake and will fight their argument through little sense but much noise.
If someone is being rude, aggressive or petty you may hear someone else advise, "Ignore him, he's a gadd."
Can be used as an adjective when someone is being critical and insesitive; "Stop being so gaddish."