na-adjective, noun
loose-lipped; talkative
Describes a person who cannot keep secrets or who talks too freely. Often used in a negative sense.
彼は口軽だから、秘密を話さないほうがいい。
He's loose-lipped, so you'd better not tell him secrets.
口軽な人に大事な話は任せられない。
You can't entrust important matters to a talkative person.