Meaning of “Gachi(ガチ)”

“Gachi(ガチ)” means seriously as a slang among young.
The word is from “gachinko(がちんこ)” which means serious match.

For a long time, “Hontou(本当)?” (which means “really?”) in youth slang was expressed as “Majide(マジで)?”. However, recently “Majide(マジで)?” has become outdated, and instead, this word “Gachi(ガチ)” is used.

Example Conversation

A: “Majide” is outdated.
B: Majide?
A: Gachi(ガチ). You are an old man.

A: “マジで”はもう古いよ
B: え、マジで？
A: ガチ。あなたはおじさん。