"'Dog' comes from Old English docga, a very rare word later used in Middle English to depict a specific, strong breed – the mastiff. …

So far, nobody has been able to reconstruct the etymological root of docga, and no ancient English word appears to be related to it.

'Dog' is therefore a true lexicological mystery of the English vocabulary."

https://theconversation.com/five-common-english-words-we-dont-know-the-origins-of-including-boy-and-dog-232299

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