A blabbermouth is someone who blabbers; they talk, talk, talk and can't keep a secret.

    嘴快、不能保守秘密的人。 
 
    例：Well, then I'll remember the next time I have something I want to keep hush-hush: Don't tell Jennifer the blabbermouth!  

    好吧，下次我守口如瓶，绝不告诉Jennifer那个大嘴巴了。