In Spain: In "Tú eres chachi", the word "chachi" is an adjective used only by boys. It means "you are cool" or "you are neat", as Cherry said. In "Tú eres una chachi", the word "chachi" could be a diminutive of "chacha". "Chacha" is an insult, and means pejoratively "housewife" or "maid", the woman who washes up or mops up.

In Mexico its common to say, "Que chido!" or "Que padre!"

Hello Andy, "chachi" means cool or neat in peninsular Spanish. If someone calls you "chachi" he is simply saying: You are cool. You are neat.