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1.
an event, undertaking, or item that seems promising but turns out to be nothing like what you imagined.
Our relationship was an upstart, she seemed really great at the beginning but it turned out she was boring and lame.
by finnegan October 17, 2005
 
2.
UpStart (uhp-stahrt) n. 1. A company or organization with innovative approaches to energy use, carbon pollution, resource consumption, and/or social equity, 2. A company or organization overcoming market barriers to build the new clean energy economy.

Coined by Lisbeth Kaufman as the title of a column on Climate Progress.
“UpStarts” are companies that are shaking up the market, breaking down barriers and helping change the economy.
by UpStarter June 03, 2011
 
3.
An enthusiastic gay male.
John was such an upstart at the protest.
by Caitlin Carlisle May 30, 2005