For some time now, some of my on-line friends have advised me to provide a version of CassidySlangScam without the invective aimed at Cassidy and his supporters. In response to that advice, I am working on providing a glossary of the terms in Cassidy’s ludicrous book How The Irish Invented Slang with a short, simple and business-like explanation of why Cassidy’s version is wrong.
The origins of the word crank, with its various meanings, are well-known. You can get a clear and interesting discussion of them here: https://www.etymonline.com/word/cranky?ref=etymonline_crossreference
It goes back to an Old English word *cranc, something bent, from a Germanic root. English retains the literal meaning of a bent or crooked thing while in other Germanic languages it has developed a figurative meaning of sick. The ‘sick’ meaning then entered English via slang. It then developed related meanings of grouchy (cranky) and eccentric (crank) or a superfan (crank). Etymologists argue about how exactly these meanings arose, though there is no doubt that all of them go back to the crooked or bent meaning of a thousand and more years ago.
Cassidy does not accept this. This is one case where Cassidy arbitrarily chooses to believe than one of these meanings comes from one Irish word, while all the rest come from a different Irish phrase.
According to Cassidy, crank in the sense of a fan comes from the Irish word crancaire, a boaster, a jester, a talker. He seems to be mixing this up with some other word, as Dinneen says that crancaire means a grumbler, and Ó Dónaill regards it as a version of cancrán, which also means a grouch or complainer.
On the other hand, all the other meanings of crank and cranky come (according to Cassidy) from the ‘Irish’ phrase crá aingí, which apparently means ‘fretful vexing, angry misery or ill-natured torment’. This is nonsense. This phrase does not exist and Cassidy offers no evidence for its existence. Aingí does mean fretful or angry or ill-natured, but these are adjectives implying feeling or emotions. Torments do not have emotions and they don’t get fretful. Nobody has ever said or written ‘Is mór an crá aingí iad’ (they are a terrible ill-natured nuisance). Cassidy copied a similar phrase from Ó Dónaill: ‘Is mór an crá (croí) iad’, but changed it to suit his purposes. We do say crá croí, of course. We don’t say crá aingí.
Even if crá aingí existed, it is a noun phrase, so it would not be used the same way as cranky and it sounds even less like crank. Even by Cassidy’s low standards of scholarship, this is total garbage.