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1.
Used to describe someone who looks as if they possess lower than average intelligence can be male or female of any age – usually found in clapped out Novas around McDonalds car parks and council estates. Can be combined with many other insults -

WARNING – If using insult directly to face – please remember they will not understand the word chromosome – or the word two –

See mouth breather
“Christ! – Look at the mouth breather in the Elisabeth Duke Clown necklace – she looks two chromosomes short of hillbilly”
by Andy The Great July 21, 2003