A Cretin is technically someone who suffers from Cretinism, a disorder which results in mental disability and stunted growth – but let’s not go there.
In every day usage a cretin is someone who is infantile, stupid and useless and irritates everyone around them.
The only good thing about cretins is that they are useful in flushing out other cretins – they’re the ones who think that the original cretin is funny.
Cretins also make up half of the people on any motorway at a given point in time – anyone who drives slower than you is a cretin, while anyone who drives faster than you is inevitably an effing moron!
“I hate getting stuck behind a cretin, we’ll miss our flight if he doesn’t speed up soon!”.