The etymology of the word ‘dog’ has always been a mystery, as there are no cognates in any other languages, even Germanic ones. Similar words, such as French ‘dogue’ (mastiff) and German ‘Dogge’ (Great Dane) have all apparently been borrowed from English. I recently came across a suggestion that it was in fact from British Celtic (Brythonic) ‘da’ = goods, property + ‘ci’ (which became ‘gi’) = dog: presumably the root of Latin ‘canis’ and English ‘hound’. So it would have meant ‘guard dog’. It is interesting that this word refers to very large dogs in French and German.