The phrase is used in reference to a person who has a vegetable shaped head, deemed displeasing to the eye. It is also used for those seen to have a bad hair style.
It derives from the use of the term “swede” – slang for head – and, as swede is one of the main ingredients of the traditional welsh broth called Cawl, the phrase became a popular put-down.
It is believed to originate from the Cefncaeau area of Llanelli in South Wales, and was commonly used among the town's young population as a derogatory term during the 1990s.
Jesus, she is ugly! Look at the swede on her... that’s a fair head for cawl!
Oh my God, look at this munter over by 'yer - fair head for cawl.