マセガキ

noun

precocious brat; sassy kid

Colloquial, mildly derogatory term for a child who acts more mature or knowledgeable than their age, often in an annoying or cheeky way.

• あのマセガキ、大人おとなみたいな口くちをきくんだよ。

  That precocious brat talks like an adult.

• 弟おとうとはマセガキで、いつも生なま意い気きなことばかり言いう。

  My little brother is a sassy kid who always says cheeky things.