A blabbermouth; a telltale. Also more fully lab of the tongue.

One who blabs; one who reveals secrets, a tell-tale.

A thing which reveals or discloses something, esp. something not intended to be made known; a revealing gesture, symptom, etc.

One who divulges: †(a) a publisher (obsolete); (b) a discloser.

figurative. Of persons; esp. one who cannot keep a secret.

One who confides.

One who betrays: in various senses of the verb.

= blabber, n.