kike (n.)

derogatory slang for "a Jew," by 1901, American English; early evidence supports the belief that it was used at first among German-American Jews in reference to newcomers from Eastern Europe, perhaps because the names of the latter ended in -ki or -ky.

There is no charity organization of any kind here [a small city in Pennsylvania] and, what is sadder to relate, the Jews in this city will not form one; that is, if the present temper of the people can be used as a criterion. The German Jews are bitterly opposed to the "Kikes," as they persist in calling the Russian Jews .... ["Report of the National Conference of Jewish Charities in the United States," Cleveland, 1912]

Philip Cowen, first editor of "The American Hebrew," suggests a source in Yiddish kikel "circle." According to him, Jewish immigrants, ignorant of writing with the Latin alphabet, signed their entry forms with a circle, eschewing the customary "X" as a sign of Christianity. On this theory, Ellis Island immigration inspectors began calling such people kikels, and the term shortened as it passed into general use.