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2.
Synonymous with complete failure despite supreme confidence. In perhaps the biggest upset in Super Bowl history, the NY Giants defeated the heavily favored New England Patriots who entered Super Bowl XLII with a perfect 18-0 record. So sure of themselves, the Patriots filed applications to patent the phrase "19-0" and "19-0 The Perfect Season" more than a week before the game, only to have their collective asses handed to them by the Giants.

see also ryan leaf
(failing a final exam despite having an A average going in) Boy, I really went "18 and 1" on that exam.

(losing the war in Iraq despite superior force going in) When will Bush ever realize that we went "18 and 1" in Iraq?
by SaMo Snatcher February 04, 2008
 
1.
to work at something and nearly accomplish something great, before failing at the end
Jimmy had an 'A' going into the final exam, but did poorly and therefore, went 18 and 1 in the class.

Entemology: The Patriots were undefeated going into the Super Bowl, but went 18 and 1 for the season.
by D. J. (from PSU) February 03, 2008
 
3.
To screw up at the end, to be imperfect
Joe thought the night was going good for him but he turned out just to be 18 and 1.
by Your Name, really February 04, 2008