In Daniel Cassidy’s worthless book of fake etymology, he claimed that the word kibosh or kybosh is of Irish origin. Cassidy was certainly not the first to claim this and his sole authority for saying it was a website called Cork Slang Online. The usual claim in relation to its supposed Irish origin is that it comes from caidhp bháis or caidhp an bháis or caip bháis, meaning a cap or cape of death. Some sources also mention cie báis, but cie is not a possible word in Irish orthography.
While caidhp bháis is given as the name of a fungus in Irish dictionaries (the death cap), there is no evidence that this is an ancient expression and it may have been composed on the pattern of the English phrase death cap in the 20th century.
There are various explanations for the meaning of caidhp bháis as a possible origin of kybosh. Some people say that it was the black cap used by a judge when pronouncing the death sentence. (I would use caipín dubh, though it doesn’t seem to be in any dictionary.) Others say that it is from the pitch cap, a punishment used by the British in Ireland where a cap of burning pitch was placed on a person’s head. This is more commonly a caipín pice in Irish. On line, I have also found claims that the caidhp bháis was a word for a candle snuffer or smóladán. There seems to be no independent evidence for any of these claims.
Only the mushroom explanation is in the dictionaries. Corpas na Gaeilge (a huge corpus of Irish material from the seventeenth to the nineteenth centuries) gives a number of examples of caidhp but nothing with caidhp bháis or caidhp an bháis.
We don’t know who first suggested this Irish origin. Charles Earl Funk said that he received this information in a letter from the poet Pádraig Colum. This is not dated but could have been in the 1930s, 1940s or 1950s. However, the earliest certain reference is in the Cornell Daily Sun from the first of December 1936, where there is an account of a lecture by a man called Conboy about the Irish origin of English words. He gives words like shanty and quid (as in a quid of tobacco, which he derives from Irish cuid, a piece) as well as kibosh.
“Kibosh,” Conboy said today, “comes from ‘caip,’ which means cap, and bais,’ which means death. “It originated in Ireland about the time of Judge Norbury, who was called the ‘hanging judge.’ When the people would see him reaching for the black cap he wore when giving the death sentence, they would say: ‘The prisoner is ‘ finished. The judge Is putting on the caip bais – kibosh. Thus when we say we ‘put the kibosh on something,’ we mean we have disposed of it.” (Editor’s note: Some authorities hold that “kibosh” might be of Yiddish origin.)
Strangely, while there is no evidence of caidhp bháis being used in the language long ago, there is certainly evidence of its existence in the language now. For example, there is this, from an article by Donncha Ó hÉallaithe in the online journal Beo in 2012:
Trí dhiúltú do na logainmneacha a bhí ar bhéal na ndaoine, rinne an Donnabhánach a chuid féin, chun caidhp an bháis a bhualadh ar an nGaeilge sa gcuid mhór den tír ina raibh an Ghaeilge in uachtar roimh an Drochshaol. (By rejecting the placenames which were in popular use, O’Donovan did his own bit to put the kibosh on the Irish language in the large area of the country where Irish was in the ascendant before the Famine.)
Unfortunately, this proves nothing. The story of the Irish origin of kibosh is so common and well-known that it is hardly surprising that people have started to use the terms caidhp bháis or caidhp an bháis in Irish in recent years. It sounds convincing and natural enough. However, without some evidence of its use in Irish before speculation about kibosh began, we can’t accept these modern uses as evidence for an Irish origin of the phrase.
There are various theories about the real origin of the word kibosh. You will find an account of these different theories by following this link: