The word hoax has a well-established origin in English. Hoax derives from an earlier word hocus, which meant to confuse, befuddle or trick someone. Hocus derives from hocus pocus, a garbled version of Latin hoc est corpus. Hocus has been around for hundreds of years, while hoax is more recent.
Cassidy doesn’t accept this. He prefers a derivation from the Irish olcas, which is pronounced olkass. (Not holkas) It doesn’t sound much like hoax. And does it mean the same thing as a hoax? No, it means badness or wickedness. Hoaxes are sometimes evil and wicked. Sometimes they are just playful. But they always involve the notion of dishonesty, of tricking people. In Irish, the words bob (as in bob a bhualadh ar dhuine, to play a practical joke on someone) or cleas (as in cleas a imirt ar dhuine, to play a trick on someone) would be the usual words for hoax. Not olcas. This is a typically fatuous and half-baked Cassidy suggestion.