Whippersnapper / (h)wipərˌsnapər / noun 

Definition: The term whippersnapper is an informal word used to describe an inexperienced and presumptuous young person. A whippersnapper is a know-it-all-kid. 

In other words, whippersnapper refers to a younger person than you, who is irritating and self-assured and shows no respect towards others, especially the older people. So, if you happen to see a young person whom you disapprove of their confident and pertinent behavior, you can refer to them as a whippersnapper.

Etymology: The term whippersnapper came around in the late 1600s when it arose out of whip-snapper, which implies a general sense of very little importance and lots of noise.

Originally, it referred to a young guy without apparent get up and go; its meaning has been altered over the years, which later used to mean an overly confident youngster.

In a Sentence

The young student is a whippersnapper who barely listens to the principal’s rules.

The older woman told her grandson that he was just a whippersnapper who could not tell her what to do.

During the meeting, the board of directors had to listen to a whippersnapper young employee who insisted on discussing the matter from the ground.

Synonyms 

Insignificancy, Stripling

Antonyms 

Eminence, Old Hand