There is no certainty about the origin of the term ‘in cahoots.’ It is first found in American English in 1829. There are two sensible theories about its origins, plus the usual ridiculous nonsense from Cassidy.
The explanation given by the Oxford English Dictionary is that English got the expression from the word French cahute, meaning a cabin or hut, which was borrowed into Scots in the 16th century. The metaphor is the same as being ‘in bed together’ – the conspirators are in a narrow space, close together. There is then a mystery about how it survived without reference for hundreds of years and surfaced in American speech in the 19th century. However, that is not so strange. Many settlers came from Scotland and it is not so strange that expressions would survive in isolated mountain communities without being written down.
However, there is another explanation, and perhaps a better one. The OED states that others have claimed an origin in the French word cohorte, the source of the English ‘cohort,’ which originally meant a band of soldiers and now means a friend or companion.
Cassidy’s claim is typically stupid and dishonest. He says that the Irish comh-údar means ‘a co-author, co-originator, co-instigator, fig. partner.’ This is one of Cassidy’s made-up definitions. In fact, comhúdar (there isn’t a hyphen in it in modern Irish) isn’t given in the dictionary, though comh (meaning joint or co-) is a common enough prefix in Irish. While údar has a range of meanings on its own, there is no evidence of anyone using comhúdar to mean anything else but a co-author of a book, document or report. Comhúdar doesn’t sound much like cahoots. If people said cahooder, he would have a point. But they don’t and comhúdar is just too wide of the mark, even if it really meant partner, as Cassidy claimed.
My money would be on the French-Scottish shed but we will probably never know for sure because the evidence simply doesn’t exist. But at least the words cahute and cohort actually existed and can be proven to have existed. There is no Irish expression comh-údar meaning a partner. It was invented by a narcissist in California about ten years ago.