cretin:
(plural: ‘cretins‘; pronunciation: the letter ‘c’ in this word is said with a ‘k’ sound)
a stupid; a foolish person; an idiot; an imbecile;
(medical) a person suffering from cretinism [cretinism = a disease in which the development of the body and mind of a person has been stopped in early childhood because of low levels (weakness) of thyroid],

What a cretin, pulling a pram behind a motorbike!
When it comes to doing something sensitive, like packing glass ware in a carton, Harry is a complete cretin; by the time he finishes packing, you only see a couple of broken glass pieces in the carton!
We were all shocked to hear that Brenda, whom we all treated as a low level cretin, saved the baby from the burning cottage without the baby getting hurt!
The manager was so upset about not finishing the project before the dead line that he called the team and its leader, a bunch of cretins.
Thanks to the advancement in the field of medicine, doctors are now able to find out about the health condition of babies even before their birth and the required treatment for cretinism is given immediately after the diagnose, and as a result, fewer and fewer cretins are born.
