Expressions (phrases, clauses, etc.), I-adjective (keiyoushi)
having a loose tongue; being talkative; speaking without thinking; being unable to keep a secret
see 口の軽い
あいつは口が軽いからこのことは言わない方がいい。Don't tell him. He's got a loose tongue.
Also written 口がかるい【くちがかるい】