North American: now usually taken as a pet form of Charles. It was originally a nickname, from an English term of endearment (as in Shakespeare's phrase ‘dearest chuck’), probably from Middle English chukken ‘to cluck’.
Irish: Anglicized form of Gaelic Ó Ceallaigh ‘descendant of Ceallach’, an ancient Irish personal name, originally a byname meaning ‘bright-headed’, later understood as ‘frequenting churches’ (Irish ceall). There are several early Irish saints who bore this name. Kelly is now the most common of all Irish family names in Ireland.