expression, i-adjective
loose-tongued; talkative; indiscreet
Describes someone who cannot keep secrets or speaks too freely. Often used critically. Compare with 口が軽い, which is the predicative form.
See also: 口が軽い
彼は口の軽い人だから、秘密は話さないほうがいい。
He's a loose-tongued person, so you'd better not tell him secrets.
口の軽い同僚にうっかり大事なことを話してしまった。
I carelessly told an important matter to my indiscreet colleague.
Predicative form meaning 'to be loose-tongued'. 口の軽い is the attributive form used before nouns.
Means 'chatty' or 'talkative' in a more neutral or positive sense, without the strong implication of revealing secrets.
Phrase composed of 口 (mouth) + の (possessive particle) + 軽い (light). The idea of a 'light mouth' suggests words come out easily, hence indiscreet.