Berk is UK slang that is generally translated as a synonym for 'fool', although it is quite a bit more vulgar. It is an example of the infamous Cockney rhyming slang, and comes from 'Berkshire Hunt'1. (or maybe 'Berkeley Hunt'2), which rhymes with c**t. It is used metaphorically, and refers to a person, not a body part.
Berk is the best known bit of Cockney rhyming slang corresponding to this word, although it is becoming a bit outdated, having been in use since at least 1936. It is also common to hear a number of celebrities' names used in the UK as rhyming slang, including Gareth Hunt, Roger Hunt, Tristram Hunt, and James Blunt; these names are used in their entirety, and not shortened to their first names as they would be in traditional cockney rhyming slang.
1. One of the oldest foxhound packs (hunting club), located in Old Berkshire Country, and commonly referred to as The Berks -- and has been for long before the lower classes sullied the word.
2. A hunting club in the west of England.