Undermango
undermango is that part of the shirt where people tuck it in or leave it out of their pants. Most of the time, or like in the old days; people had the custom like nowadays in many sports to tuck the undershirt part known as today the "undermango" shirt inside their pants, as they say; generates more style and some say; comfort.
instead of saying my undershirt part tucked in, you can say instead: tuck my Undermango in.