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 and Dutch: from the personal name Derrick (now more commonly spelled Derek in England, earlier Dederick), which was introduced to England in the 15th century, from Dutch Diederick, Dirck (see Terry).