Top definition
A foolish and fairly stupid person, usually online, who is generally completely oblivious to how much of an idiot they come off as. They usually have poor grammar, bad social skills and there's seldom anything thought provoking or even remotely interesting about their opinions and/or they post things that are ridiculously off topic to the discussion at hand.

There are two flavors of them: the young and immature garden variety Dunks and then the trollish, masochistic lifelong ones with something seriously wrong with them emotionally and mentally. Most Dunkasses are often young and usually mellow out as they get older and learn better netiquette, some even becoming valued and productive members of the community in question, though a rare few, people like Christian Weston Chandler, will stay completely unrepentant, stubbornly refusing to learn from their mistakes and learn social skills. A lifelong Dunkass often ends up with a long list of communities from which they've been banned and a bad reputation across their fandom or community.
Sam just posted something so stupid on our message board, he's such a Dunkass, we may have to ban him if he doesn't grow the fuck up.
by ShinCookie November 02, 2016
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2
Someone who is very tall for their age/race/sex, but is none-the-less incredibly useless at everything but b ball.

Coined by Sweet Bro & Hella Jeff authour Andrew Hussie
God that dunkass Gilbert is only good for his slamdunk skills.
by Trompe la Mort September 29, 2011
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3
Someone who forgets things often, leading to very problematic outcomes. They are also slightly or very incompetent.
"John's a total dunkass, he forgot to sign out of his g-mail and now he's getting daily letters from adult dating websites."
by TheRickestRick May 26, 2016
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