I always thought the reason why everyone called young Arthur "Wart" in the book was because he was the ward (a minor placed under the protection and care of a guardian) of Sir Hector. And, over time, the child (who was viewed so indifferently by his guardian that he wasn't even referred to by his proper name), came to be called "Wart" as a lazy mispronunciation of "the ward".
-- Nanaea