Old, baggy, billowy underpants that you always wear on inappropriate occasions.
Poor Phil got changed in the mixed lockers at the gym today but forgot he had his Bellpants on.
White underpants made of cotton that fall below the knee making the wearer look a bit of a chav.
Bellpants are pants that you can wear on their own around the nostalgic city of Bath. Very common to find 40 year old men wearing only these pants outside the famous Francis Hotel in Bath.