Originally an Anglicized form of the ancient Irish male name Ceallach. It is now very widely used in the English-speaking world, mainly as a girl's name. This is a transferred use of the surname Ó Ceallaigh ‘descendant of Ceallach’.
German, Dutch (also de Klein(e)) and Jewish (Ashkenazic): from Middle High German, Dutch, German klein ‘small’, or Yiddish kleyn. This was a nickname for a person of small stature, but is also often found as a distinguishing name for a junior male, usually a son, in names such as Kleinhans and Kleinpeter. This name is common and widespread throughout central and eastern Europe.