Definitions for tank top
Random House Webster's College Dictionary
a low-cut, sleeveless, pullover shirt with shoulder straps, often made of lightweight knitted fabric.
Origin of tank top:
a tight-fitting sleeveless shirt with wide shoulder straps and low neck and no front opening; often worn over a shirt or blouse
An item of casual clothing: a top with no sleeves.
The top of a ship's bilge tank, usually the lowest horizontal surface on board.