タメ口ぐちをきく

expression, godan verb (-ku)

talk casually; use informal language

Refers to speaking in a casual, peer-like manner, often when the situation normally calls for polite speech. タメ口 is slang for casual speech among equals.

See also: タメ口

• 先せん輩ぱいにタメ口ぐちをきくのは失しつ礼れいだよ。

  It's rude to talk casually to your senior.

• 彼かれは先せん生せいにまでタメ口ぐちをきく。

  He even uses casual language with his teachers.