gormless means Lacking intelligence and vitality and dull a slapper is someone who puts it out more than they ought. A gormless slapper is dumb and will root anyone
This is a rare phenomenon, which can happen when a turd is so solid it stands up straight, only to collapse, hit the balls and destroy everything in it's path.
As a fellow Dutchman I can say that these toilets are notoriously known for causing so called 'dutch bag-slappers"
dutch bag slapper