There is a pair of interesting words in Daniel Cassidy’s incredibly stupid book, How The Irish Invented Slang, the words geek and kike. Geek is defined by Cassidy as a fairground term for a long-haired person, which is pushing it – the geek was frequently the ‘wild man’ in a fairground (think of Sideshow Bob on the Simpsons) but this seems to be a relatively late meaning. The word kike is a racist insult used by ignorant bigots to refer to Jews. The reason why they are placed together in this post is because Cassidy derived both of them from derivatives of the Irish word ciabh, meaning tress or curl.
Cassidy’s claims about these words are just as absurd as the rest of this crazy book.
Leaving aside the fact that ciabhóg and ciabhach are quite similar but they supposedly gave rise to two words which sound very different in English, and that neither word sounds much like either geek or kike, it is a measure of how random and arbitrary Cassidy’s Irish explanations for American slang were that he decided to assign kike to ciabhóg but geek to ciabhach. If you accepted an Irish derivation for these words, it would be just as reasonable to argue for it the other way round, with kike coming from ciabhach and geek coming from ciabhóg, or derive both from ciabhach or both from ciabhóg, or derive one or both from caidheach (a bit like kayak but with the final k sound like the ch in Scottish loch) which means filthy or from caoch meaning blind or from gíoc (pronounced geek) meaning a squeal or a chirp. I’m not actually suggesting that any of these are the origin of these terms. I am merely showing how unreliable and shoddy and dishonest Cassidy’s methods were and how unlikely it is that methods like this will reveal anything of value, even by accident.
Back in the real word, geek is from a Germanic word meaning a fool, geck, which was actually used in the 18th century in the Austro-Hungarian empire for people in travelling freak shows. As for kike, there are various explanations, the most likely of which is that it comes from the Yiddish word for circle, because illiterate Jewish immigrants used to sign with a circle rather than with the cross used by Christian immigrants.