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To bluehorn something. To completely screw something up in the last second, even though it was pretty much a guaranteed success already.

Has its origin from the series finale of HIMYM, that was described by many people as one of the worst (if not the worst) in recent TV history because of its unnecessary and mostly unwanted plot twisting, which a blue french horn is symbolic for in the shows legacy. Hence to bluehorn something.
"Manchester United completely bluehorned it in the overtime."

"I loved Dexter, but the last season and the finale was such a bluehorn."
by w4yland October 10, 2014
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