A person (normally a farmer) is someone who tends to sheep’s coats and shaves them for wool. It comes from the term shag which refers to the sheep’s wool that hasn’t been shaved off.
It is also widely known as Brandon or Brenda the sheep shagger.
The saying comes from a young boy from the north of wales who one day found out his mum was stuck in a washing machine decided to venture out, but went down a different path, on that path he came across a sheep and decided to fuck it resulting in the saying sheep shagger