Definition of "upstart"

upstarts […] they call in reproach mushrooms

1631, Francis [Bacon], “VI. Century.”, in Sylua Syluarum: Or A Naturall Historie. In Ten Centuries. […], 3rd edition, London: […] William Rawley; [p]rinted by J[ohn] H[aviland] for William Lee […]

[S]he has no fair pretence of family or blood. She was nobody when he married her, barely the daughter of a gentleman; but ever since her being turned into a Churchill she has out-Churchill’d them all in high and mighty claims: but in herself, I assure you, she is an upstart.”

1815 December (indicated as 1816), [Jane Austen], chapter XVIII, in Emma: […], volume II, London: […] [Charles Roworth and James Moyes] for John Murray, page 345

But electric vehicles and the batteries that made them run became ensnared in corporate scandals, fraud, and monopolistic corruption that shook the confidence of the nation and inspired automotive upstarts.

2006, Edwin Black, chapter 1, in Internal Combustion‎

Where the Czech upstart [Lukáš] Rosol, ranked 100 in the world, all but blew [Rafael] Nadal's head off with his blunderbuss in a fifth set of unrivalled intensity on Thursday night, [Julien] Benneteau, a more artful citizen, used a rapier to hurt his vaunted foe before falling just short of a kill. In the end, it was he who staggered from the scene of the fight.

2012 June 29, Kevin Mitchell, “Roger Federer back from Wimbledon 2012 brink to beat Julien Benneteau”, in The Guardian‎, archived from the original on 15 November 2016

Th' Elfe therewith astownd,Upstarted lightly from his looser make,And his unready weapons gan in hand to take.

1590, Edmund Spenser, “(please specify the book)”, in The Faerie Queene. […], London: […] [John Wolfe] for William Ponsonbie

the beauteous beast Scared by the noise upstarted at our feet

1859, Alfred Tennyson, “(please specify the page)”, in Idylls of the King, London: Edward Moxon & Co., […]

"Be that word our sign of parting, bird or fiend!" I shrieked, upstarting—

1845 February, — Quarles [pseudonym; Edgar Allan Poe], “The Raven”, in The American Review‎, volume I, number II, New York, N.Y., London: Wiley & Putnam, […]