In the days of sail smooth bore black powder cannon were named for the weight of cannonball they fired. A 6 pounder shot a 6 pound round cannon ball.

This nomenclature continued after the switch to rifled cannon and cylindrical ammo (though meant less and less as time went on). The weights of various weapons was standardised.

So a 2 pdr (40mm bore) nominally fired ordinance weighing 2 pounds, a 6 pdr (57mm bore) fired 6 pound rounds, etc.