ダチ (dachi) is Japanese slang for friend, buddy, mate. It comes from chopping 友達 (tomodachi) in half and keeping just the ending. That's how slang works everywhere: take a perfectly good word, hack it apart, and suddenly it sounds cooler. You won't hear this in a business meeting. ダチ (dachi) lives in manga speech bubbles, late-night izakaya conversations, and the kind of friendships where formality would feel weird. It's the word you use when someone's got your back, no questions asked.