Another oft-quoted piece of fantasy from Daniel Cassidy’s ridiculous book, How The Irish Invented Slang, is the claim that the English word fluke, meaning a stroke of luck, comes from the Irish fo-luach.
Cassidy claims that fo-luach means “a rare result, a rare reward, a rare payment, an occasional payoff”. Fo-luach (or foluach) does not exist in any dictionary or in any Irish text. If it existed, it would be a translation of ‘a subsidiary value’. Irish has several words for a windfall, of which amhantar would be the most common. Cassidy invented the ‘rare reward’ meaning by taking the most obscure dictionary definitions of the prefix fo- and the word luach and combining them. It is a total fabrication. Somebody well-disposed towards Cassidy would maybe call it an unwise and over-enthusiastic claim. Personally I think it’s just a bare-faced lie, like nearly all of Cassidy’s ‘research’.
The real origin of fluke isn’t known. Some of the different theories are examined here:
cassidyslangscam 