Cantankerous is an interesting word. It means crabbit, peevish, disgruntled.The Online Etymological Dictionary says that it occurs first in English in 1772, that it is said to be “a Wiltshire word,” and that it is ‘probably from an alteration (influenced by raucous) of Middle English contakour “troublemaker” (c.1300), from Anglo-French contec “discord, strife,” from Old French contechier (Old North French contekier), from con- “with” + teche, related to atachier “hold fast” (see attach).’ The Oxford English Dictionary says that it is “perhaps a blend of Anglo-Irish cant ‘auction’ and rancorous (see rancour) With -ous.’
I am not particularly keen on these suggestions (the dictionary dudes do their best but there are lots of words and it must be hard) but they certainly make more sense than Cassidy’s suggestion that the word derives from ceanndánacht ársa, which Cassidy defines as ‘old obstinacy, aged wilfulness, elderly stubbornness.’ This is nonsense because this is a noun phrase, while cantankerous is an adjective and also because the ársa primarily means ancient as in long, long ago. Admittedly life was probably hard back in the days of yore but I’m sure our ancestors lightened up occasionally. It certainly doesn’t mean ‘typical of old people’, which is the meaning Cassidy had in mind here. Of course, nobody in Irish ever talks about ‘ancient stubbornness’ when they talk about the miserable old biddy next door who is always complaining. The phrase originated with Cassidy, who didn’t speak any Irish and didn’t know what he was talking about so why would anyone place any trust in what Cassidy said?
My own personal belief (and I may well be completely wrong) is that cantankerous is a blend of the words contentious and cankerous, possibly influenced by Irish words like cancrach, cancrán, which are related to words like cankerous (either as borrowings from English or from French) and show a different meaning of these words. Cancrán is defined by Ó Dónaill as ‘cantankerous person or crank’. I don’t think cantankerous comes directly from Irish because there is no such word as cantancarán and cancrán sounds very different, though it is certainly the word you would use of a grouchy person, so the meaning is a really good match.