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1.
A phrase used to describe an individual with extremely low intelligence. A generally unintelligent person may simply be described as 'thick', however the term 'thick as shit' describes someone who is so dumb that it may be considered reasonable to question if they're mentally retarded. Such people's intelligence is often so low that persons of normal intellect may find it incomprehensible how they can be so stupid.

Someone may be considered/accused of being 'thick as shit' for several reasons, mainly for thinking and acting in ways which most people would obviously recognise as wrong/stupid/flawed/absurd. Unlike people of higher intellect, such people will be unable to realise how hopelessly dumb they really are even after it has been clearly shown to them. These people are unable to understand simple logic and have very poor communication skills.

The term is typically used informally and is commonly used as an insult intended to cause offence.
"He must be thick as shit if he thinks the moon's made of cheese"
"Ha Ha, you're thick as shit!"
"I know someone who ate cyanide because he's thick as shit"
"That person is obviously thick as shit"
by dictionaryman2007 September 02, 2007