Cassidy claims that the word slugger, as in someone who hits a ball strongly, comes from the Irish word slacaire, which means a hitter or a batsman (slacaí in modern Standard Irish). On the face of it, this seems like a convincing claim but if you examine it more closely, it starts to lose its appeal.
Firstly, let’s look at slug as a word for hitting. The English dictionaries agree that it is related to slog (as in ‘a hard slog’) and that its origin is unknown. For the word slog, its first recorded use in English is apparently in 1824, while slug is even later, in 1830.
The Irish word slacadh apparently comes from slaic (slacán) which mean stick or bat. So far, so good for Cassidy’s theory.
The problem is this. Slog looks Germanic. It looks and sounds as though it is related to the German schlagen or the Yiddish shlogn. It may well be an English dialect word which went mainstream around the time that dialectologists first went to work. Whatever the real origin of slog/slug, I don’t believe in Cassidy’s theory.
My reason for being sceptical of Cassidy’s claim is that slacadh sounds very different from slog or slug. The sl is the same, but if English borrowed the word slacadh without changing the sound, you would have a great batsman described as a slacker, who really slacks the ball! Slog could have been borrowed into English from Irish, but it means to swallow. When languages borrow from other languages, they frequently have difficulties because the other language has sounds or combinations of sounds that are unfamiliar and then they change the borrowed word. This wouldn’t be the case here. Both slack (slac) and slog (slog) are perfectly good words in both English and Irish, so there would be no reason to use slog instead of slack. Admittedly, slack has another meaning in English and this could be confusing, but then lots of words have lots of different meanings. People don’t assume that someone has given up football because they kicked the ball or that someone has murdered his wife because he took her out last night. Most words and phrases are complex and multi-faceted but native speakers still manage to use them effectively. I don’t believe that anyone would have a problem with “slacking the ball” if it existed as a common phrase, but it doesn’t. People say slug, not slack, and they do that because they didn’t borrow it from Irish slacadh.
I admit that this is somewhat more plausible than most of Cassidy’s claims, but I am still unconvinced because of the lack of any phonetic similarity.