The word swank means dashing smartness or swagger. It is an English dialect word, related to swing. The root of it is the notion of swaggering, and from this it has picked up other notions of self-importance or poshness. It is a cognate of the German word schwanken, meaning to sway, to oscillate, to vary.
Daniel Cassidy, in his idiotic How The Irish Invented Slang, chose to ignore the available information on the origins of the word and claimed instead that it derives from the Irish word somhaoineach. This is obviously nonsense, even on the basis of pronunciation. The word somhaoineach has three syllables, while swank has only one. Like many English words, swank has a clipped, Germanic sound to it while somhaoineach is soft and Celtic. It is pronounced soh-ween-yah. It would not become swank in English.
It is a very, very rare word, derived from the word maoin meaning wealth or property and the particle so– which means easy, along with the adjectival ending –each. Ó Dónaill defines it as ‘profitable, valuable’, which is barely related to the various meanings of swank and swanky.
In short, there is no evidence for Cassidy’s claim and there is plenty of evidence in favour of the English derivation.