Another of the crazy and ridiculous derivations in Daniel Cassidy’s book How The Irish Invented Slang is his speculation about the origins of the word doggone. It is worth giving Cassidy’s entry on this subject in full, as it shows clearly how fraudulent and incompetent Daniel Cassidy was.
Doggone, excl., adj., adv., an exclamation of irritation, disappointment, someone or something nasty, crude, gross; darned. Not recorded in English till the 19th century. Origin unknown.
Dogairne, n., a gross crude person or thing. Dógan, n., a sort of oath or exclamation, (Ó Dónaill, 1977, 427; Dwelly, Gaelic-English Dictionary, 1901, 347.)
Let’s just examine Cassidy’s claim carefully. First of all, Cassidy says that the origin of ‘doggone’ is unknown. Is it?
Of course not! As usual, Cassidy was lying.
The dictionaries are agreed that dog-gone or doggone is a 19th century Americanism and that it is a minced oath, a disguised blasphemy. The Oxford English Dictionary says it is ‘generally taken as a deformation of the profane God damn.’ The American Heritage Dictionary of the English Language (4th ed.) says that ‘doggone’ is an ‘alteration of the Scots dagone,’ which is in turn an ‘alteration of goddamn.’ Merriam-Webster says that it is ‘a euphemism for God damn.’
Furthermore, God damn and doggone are used in exactly the same way. Sometimes they are an exclamation, God damn it! (Doggone it!) What have you done? and sometimes as an adjective, Just take the goddamn (doggone) money already!
It is an indication of how stupid and deluded Cassidy was that he seems to think giving two derivations strengthens his case. Of course, in reality, the fact that Cassidy provides two completely separate words in different languages just serves to show how easy it is to find a spurious Gaelic derivation.
Dogairne is a rare word derived from docair (more usually deacair in modern Irish), which means ‘hard, difficult’. Dogairne is a noun, and is defined as ‘A gross, crude, person or thing.’ Doggone, of course, is not used as a noun. You can’t say ‘My cousin is a total doggone.’
As for dògan, this is a Scottish Gaelic exclamation. The Scottish Gaelic dictionaries make it quite clear that this is a borrowing from doggone or from the Scots equivalent. Note also that Cassidy once again betrayed his total ignorance of the Gaelic languages, as he wrote it as dógan. Anyone who knew the slightest thing about Irish and Gaelic would know that Scottish Gaelic always uses grave accents (the ones that slope back) while Irish always uses acute accents (sloping forward).