A plug for closing a neck of a carboy is disclosed. The plug comprises a cap for obturating the neck and a skirt for attachment to the neck. The skirt includes a first skirt portion, provided with a bulging line for interfering by jamming with the neck, and a second skirt portion, adapted for substantially encircling snugly an outer face of the neck. In order to avoid the presence of liquid between an outside of the neck and an inside of the skirt after placement of the plug, an inner face of the second skirt portion is provided with draining means adapted for delimiting fluid flow passages between the second skirt portion and the neck, which connect with each other opposite axial ends of the second skirt portion.