The term nerk, as used in Porridge was a cleaned up version of berk.  Berk had to be cleaned up because it was an abbreviation of Berkshire Hunt which was rhyming slang.  Pedant alert.  As was calling somebody a Charlie or a proper Charlie.  That was a cleaned up version of Charlie Smirke ( a very famous jockey in his day) which also was a sanitized version of Berkshire Hunt.