From Longman Dictionary of Contemporary Englishupstartup‧start /ˈʌpstɑːt $ -ɑːrt/ noun [countable] BOASTsomeone who behaves as if they were more important than they really are and who shows a lack of respect towards people who are more experienced or older a cheeky young upstart —upstart adjective
Examples from the Corpus
upstart• We made suggestions, to be overruled by upstarts.• She certainly hoped Albert didn't let in any little upstarts like the officer who'd wanted her apartment!• Well, she wasn't about to let this little upstart have her home.• These upstarts are beating us at our own game, manufacturing and marketing.Origin upstart (1500-1600) upstart “to jump up suddenly” ((14-19 centuries)) 