To set out to accomplish a life task, only to fail miserably right off the bat. Usually done so in a humerously ironic manner that will make great campfire stories later on in life.
Examples of a Costanza would include, but are not limited to:
Spilling hot chocolate on the girl you've just met, thus giving her 3rd degree leg burns and forever friending you by default.
Falling into a gaping lawn hole while attempting to act suave whilst playing hot potato with various clothing items.
Attempting to engulf any sort of hot vegetable whole without the aid of silver ware, thus creating an awkward situation along with severe tongue injuries.
Sitting in a room, petting a dog's ass for 3 hours while your friends make moves on the girl you came there in the hopes of getting to know.
Being caught with dubious amounts of contraband by an insane lawyer, then proceeding to forge a night's worth of false documents in order to prove your innocence.
Chris: "So, you think that girl I met the other day will be there tonight? I really hope so, because she seems pretty cool."
Kyle: "Yeah man, just don't screw this up. Play it cool, you'll be fine."
10 Minutes after arriving to her house....
*Chris spills hot chocolate all over the girl's leg, and swallows a potato whole, then proceeds to pet the dog's ass the rest of the night out of shame and confusion*
Kyle, Mike, Chris: "COSTANZA!!!! CO-STANZA!!!"