This is another absurdity and another example of Cassidy’s selective treatment of his sources. Cassidy says that dock, meaning to take a chunk out of someone’s wages as a punishment, comes from the Irish tobhach, which means a levy. Tobhach is pronounced toe-akh or toe-ah, so it doesn’t sound a lot like dock anyway, but it would not be an entirely unreasonable suggestion if there were no better candidate.
And this is Cassidy’s claim, that there is no other candidate, that dock suddenly appears in English out of the blue in the 19th century. But this is a distortion of the truth. Cassidy cherry-picked the information in the English dictionaries and only used what made his case look stronger (while insulting the hard-working lexicographers whose work he parasitized). Dock in the sense of taking a chunk out of your pay only goes back to the 1820s but this is merely a natural extension of the meaning of a word which has been used since the Middle Ages in English to describe the action of cutting an animal’s tail off. All dictionaries recognise that dock (cut off a tail) and dock (clip someone’s wages) are the same word. And if this word has been used in rural communities in England in the tail-cutting sense since at least the fourteenth century, it doesn’t come from Irish. QED.
This is a perfect example of how Cassidy distorted the facts in an attempt to make a case for derivations which are nonsense.