An upstart is a person who has come up fast — and who acts far more important than they have earned. The word is an insult, and the insult has two halves. First: this person is new. They have not been here long, they have not done the years. Second: they show no respect to the people who have. A young manager who talks over the senior staff, a new company that mocks the old ones, a junior player who tells the veterans how to play — all upstarts, in somebody's eyes. That last phrase matters: in somebody's eyes. This word tells you as much about the speaker as about the person described. It is what the established side calls the newcomer. So there is often an old-fashioned, slightly grand flavour to it, and sometimes the speaker is simply annoyed at being challenged. A teacher should say this plainly: when you hear 'upstart', ask who is offended, and whether the newcomer is actually wrong. It works as a noun ('who is this upstart?') and as an adjective before a noun ('an upstart company', 'some upstart lawyer'). In business writing it has softened a little — 'upstart airline' can mean simply 'bold new rival' with some admiration mixed in — but the sting is still there underneath.