upstart; mushroom millionaire
Refers to someone who has suddenly risen to wealth or prominence, often with a nuance of lacking refinement or old money status. Somewhat dated; more common in literary or historical contexts.
He is an upstart who made his fortune in one generation.
In old novels, nouveau riche characters derided as upstarts often appear.
Compound of 出来 (deki, 'result, outcome') and 星 (hoshi, 'star'), literally 'star that has appeared'. The exact historical derivation is uncertain, but it metaphorically refers to someone who has suddenly risen to prominence.