tight as a drum
Taut or close-fitting; also, watertight. For example, That baby's eaten so much that the skin on his belly is tight as a drum, or You needn't worry about leaks; this tent is tight as a drum. Originally this expression alluded to the skin of a drumhead, which is tightly stretched, and in the mid-1800s was transferred to other kinds of tautness. Later, however, it sometimes referred to a drum-shaped container, such as an oil drum, which had to be well sealed to prevent leaks, and the expression then signified "watertight."