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1.
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."
by Mal de Fées December 03, 2012
 
2.
An object that has become lodged in ones anus
Oh no, I have another gadd. Off to the hospital I go.
by Wayne S. Thompson January 04, 2011
 
3.
Guy/Girl Attention Deficit Disorder.

Any person male or female who is characterized by such an extreme form of A.D.D. that the disorder transfers over to s/he's dealings with love. In other words the person cannot sufficient satisfaction from one potential mate nor maintain focus on the said person, therefore they must constantly drift from one lover too another. Essentially a womanizer/manizer/player but they have the ability to blame their actions on their 'disorder'.
Woman suffering from G.A.D.D. speaking to her 'boyfriend': When I fell outta the boat- oh by the way did I tell you the boat's name was Ol' Jim and my father names all my boats. But about falling outta the boat... OH HEY (glancing a man who just walked in... I'll be back in a little bit (runs off to the other man and immediately engages in flirtatious behavior and later repeats her action with another man)

'boyfriend' (muttering himself): Well so much for that... she needs something more than just Ritalin for that G.A.D.D.
by Victim of a G.A.D.D. woman May 20, 2009
 
4.
Guy Attention Deficit Disorder, basically, when a girl can't keep her interest in one/concentrate on one guy.
She's got GADD so bad, three boyfriends isn't enough
by Jess Rodchester November 08, 2004