I have already said that Cassidy ignores perfectly good English explanations for words in favour of improbable or impossible made-up Irish derivations. This is a perfect example. Chicken means scared and a chicken is a coward. I think this comes from the English word chicken which is a nervous type of bird. In English, phrases like hen-hearted go back to the 14th century at least. It is obvious, realistic, and it ticks all the boxes.
Cassidy and his supporters will have none of it. Chicken doesn’t come from chicken, apparently. It comes from teith ar cheann, which means – says Cassidy – to run away first. Does it? No, of course not. This is How The Irish Invented Slang we’re talking about here, not a serious work of scholarship! Teith ar cheann is unattested. If you look it up on Google, you will find a handful of references to Daniel Cassidy. In terms of Irish grammar, it doesn’t make sense, as it really means ‘flee at the head of’ rather than flee first. At the head of what? I hear you ask. Exactly. On its own, this phrase means nothing.
There are lots of expressions for a weakling or coward in Irish and any of them could have been used in slang, so it seems strange that people would use a grammatically meaningless and unfamiliar phrase in preference to these words. Of course, in reality, they didn’t. Chicken is English. A chicken is a chicken is a chicken. And Cassidy was a birdbrain.