also: ぺいぺい · ぺえぺえ
noun
Colloquial, often self-deprecating or derogatory term for a low-ranking, inexperienced person in a workplace or group. Used humorously or humbly about oneself, or dismissively about others.
入社したばかりのペーペーです。
I'm just a newbie who just joined the company.
ペーペーのくせに生意気だ。
He's just a small fry but acts so cocky.
The origin is uncertain. Possibly a reduplicative mimetic word suggesting something trivial or insignificant, or derived from the sound of a baby babbling, implying immaturity.