Hertfordshire Brown is a phrase commonly used in the regions outlying northern London to describe a bog-blocking shit.
Mat: I've laid another Hertfordshire Brown!
Nick: You shouldn't eat so much spam then you fucking moron.
A freakishly large, hairy, brown spider that is known to live in the Hertfordshire region of the UK. It looks like a tarantula, and is yet not one. They are big scary mofos that refuse to die!
"Mum there's a hertfordshire brown in my room!"
"Don't look at me! That thing won't fit in a pint glass!"