also: ためぐち
noun
casual speech; informal language; peer language
Refers to speaking in a casual, informal manner, typically used among close friends or equals, without honorifics or polite forms. Often contrasted with 敬語 (keigo).
先輩にタメ口を使うのは失礼だ。
It's rude to use casual speech with your seniors.
彼とは仲がいいから、いつもタメ口で話している。
We're close, so we always talk in casual language.
From ため (same, equal) + 口 (speech, way of talking). The word ため itself is a slang term meaning 'equal' or 'same age', possibly derived from 揃い (そろい) or a reversal of めだ (目だ).