Someone who may be intelligent, but who does something classically stupid which would infer that he/she is in actuality unintelligent. Someone who acts negatively and out of character and is reprimanded, in jest, by their peers. Similar to the UK's "wanker", but used more lightly and with humor in Canada.
Cheating on a Hot, Sweet Girlfriend / Climbing Buildings while intoxicated with loafers / Peeing too close to metal in -28 weather / Deciding it's a good time to explore your emotional, feminine side at the Urinals in the Club / Getting Caught with exam answers in micro-print, taped to your HB pencil... Female: Putting Nair on your Eyebrows / Using Dish-detergent in the Clothes Washer / Never Checking the Oil / Video-taping the Stagette / Letting that Hot guy from the Hockey-team video tape slightly-tipsy porn-star-you...
"Wow, 'Johnny'/'Jenny', that was a Dummm Asss thing to do; you are a Dumass!"