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.
Flunky (or flunkey) seems to be a Scots dialect word. The Online Etymological Dictionary has this to say (https://www.etymonline.com/word/flunky)
flunky (n.) also flunkey, 1782, Scottish dialect, “footman, liveried servant,” of uncertain origin, perhaps a diminutive variant of flanker (in reference to servants running alongside coaches; compare footman). Sense of “flatterer, toady” first recorded 1855. “Recent in literature, but prob. much older in colloquial speech” [Century Dictionary].
I would be surprised if the word monkey did not play a role in influencing the sound of this word too, though that is just a personal opinion.
Daniel Cassidy, in his work of false etymologies, How The Irish Invented Slang, claims that it comes from the Irish fulangaí, which he defines as:
Fulangaí (pron., fulǝngí), n., a prop, a support (for a person), someone who props up and supports a champion (or a chief); fig. a footman or attendant; a patient, passive, long-suffering, enduring person. (Dineen, 500; Ó Dónaill, 594)
Dinneen’s dictionary has the following:
Fulangaidhe – a sufferer; a prop or support, one who props up, a champion.
Ó Dónaill says this:
fulangaí1, m. (gs. ~, pl. -aithe). 1. Sufferer, long-suffering person. 2. (Of person) Prop, support.
Spot the difference? Neither of the dictionaries Cassidy quotes mentions the meaning of ‘a footman’. The sound of fulangaí (fulangaidhe in the pre-reform spelling) isn’t much like flunkey either (ful-an-gee). Cassidy’s claim is highly improbable.