Kike Leo Rosten, The Joys of Yiddish:
The word kike was born on Ellis Island when there were Jewish migrants who were also illiterate. When asked to sign the entry-forms with the customary “X”, the Jewish immigrants would refuse, because they associated an X with the cross of Christianity. Instead, they drew a circle as the signature on the entry-forms. The Yiddish word for “circle” is kikel (pronounced KY-kul), and for “little circle”, kikeleh (pronounced KY-kul-uh). Before long the immigration inspectors were calling anyone who signed with an “O” in place of an “X” a kikel or kikeleh or kikee or, finally and succinctly, kike.