whippersnapper

/ˈwɪpərsnæpər/

whip·per·snap·per

BrE /ˈwɪpəsnæpə(r)/

NAmE /ˈwɪpərsnæpər/

(old-fashioned, informal)

a young and unimportant person who behaves in a way that others think is too confident and rude