Altered short form of William, not used before the 19th century. The reason for the change in the initial consonant is not clear, but it conforms to the pattern regularly found when English words beginning with w- are borrowed into Gaelic. The nickname ‘King Billy’ for William of Orange is an early example from Ireland, which may have influenced English usage. It is bestowed occasionally as a name in its own right.
English: nickname, most likely for a tall, thin man with long legs, from Middle English cran ‘crane’ (the bird), Old English cran, cron. The term included the heron until the introduction of a separate word for the latter in the 14th century.