Swell is (or was) a common American expression. It originally meant posh, upper-class and then acquired the meaning of ‘wonderful’. The excellent Online Etymological Dictionary says this about it as a verb:
“Old English swellan “grow or make bigger” (past tense sweall, past participle swollen), from Proto-Germanic *swelnanan (cf. Old Saxon swellan, Old Norse svella, Old Frisian swella, Middle Dutch swellen, Dutch zwellen, Old High German swellan, German schwellen), of unknown origin.”
As a noun, Etymonline says this:
“early 13c., “a morbid swelling,” from swell (v.). In reference to a rise of the sea, it is attested from c.1600. The meaning “wealthy, elegant person” is first recorded 1786; hence the adjectival meaning “fashionably dressed or equipped” (1810), both from the notion of “puffed-up, pompous” behavior. The sense of “good, excellent” first occurs 1897, and as a stand-alone expression of satisfaction it is recorded from 1930 in American English.”
The development here is pretty clear. Daniel Cassidy, in his insane book How The Irish Invented Slang, chooses to ignore the opinion of language experts and link it to the word sóúil in Irish. Sóúil is a real word (many of Cassidy’s phrases are made-up) but it is not at all common. It is pronounced soh-ool, which is not a lot like swell. It means ‘well-off, comfortable, delicious’. There is no evidence in favour of Cassidy’s claim and the most parsimonious (i.e. the least fanciful) explanation is that swell in the sense of good developed out of the earlier senses of swell as fashionable or posh. After all, people used to talk about dandies, and then they started saying ‘that’s just dandy’. And the Irish mórtasach means ‘puffed-up, swollen, proud or haughty,’ which shows a similar development of meaning to the English, as does the Irish word borr (‘swell’) which can also be used to describe someone being as swollen-headed or full of themselves as Daniel Cassidy was.
Cassidy uses a typically lame argument to dismiss this alternative etymology. “Swells are not swollen; they are sóúil (prosperous, wealthy, elegant). A swell desert is not a protuberance; it is sóúil (delicious). And a swell gal isn’t bloated, she’s cheerful and grand.”
And Hollywood stars don’t twinkle in the night sky, and you can’t cut yourself on a sharp dresser. So what? Language is full of metaphor.
I wonder which tasty desert Cassidy was referring to here? Sahara? Gobi? Atacama? It reminds me of the recurring dream I have when I’m on a diet. I am stranded on a dessert island with a spoon, surrounded by shark-infested custard. Gradually, I eat my way to the brink of ecological disaster. At the end, I’m sitting on a metre-wide platform of tiramisu and I’m still eating …