タメ語
Tamego
ためご
'Tame' (meaning 'equal/same age') + 'go' (language) refers to speaking without keigo (polite/formal language). Originally it meant the way same-age peers talk to each other, but now it broadly means any casual speech style regardless of age. 'Tame-go de ii yo' ('You can use casual speech') is the go-to phrase for closing social distance. A similar term is 'tame-guchi' (casual tone/mouth).
Examples
もうタメ語でよくない？敬語だと距離感じるし
Can't we just drop the formalities already? Using keigo feels so distant.
Between acquaintances who've known each other for a while
バイト先の先輩がタメ語でいいよって言ってくれた
My senpai at my part-time job told me I could use tame-go with them.
Telling a friend about their part-time job
初対面でいきなりタメ語の人、ちょっと苦手
I'm not great with people who use tame-go right from the first meeting.
日本語学校の先生にうっかりタメ語使っちゃった
I accidentally used tame-go with my Japanese school teacher.