It likely originated in rhyming slang where words that rhyme are used as replacement[0]. It isn't the only example in nick names. Bill is a nickname for William

It likely originated in rhyming slang where words that rhyme are used as replacement[0]. It isn't the only example in nick names. Bill is a nickname for William and Bob is a nickname for Robert. [0] https://en.wikipedia.org/wiki/Rhyming_slang https://en.wikipedia.org/wiki/Rhyming_slang