Anyone who has studied English language knows the words ‘dog’ and ‘God’. Now, some researchers claim that the first word was coined from nothing in contempt of God, being ‘God’ spelled backward.
It may seem far-fetched, but the genesis of ‘dog’ is unknown. The English tongue, we are told, is the fusion of a Germanic linguistic structure, the Anglo-Saxon, enriched by Latin and Frankish contributions through the Normans.
However, in the Latin-derived languages, ‘dog’ is similar to the Latin root ‘canis’ and in the Germanic languages, in general, it is called hond / hund (related to English ‘hound’). The most common theory is the following. Several centuries ago, a ferocious canine breed was created and called ‘dog’ and, over time, this name was extended to all dogs in place of the term ‘hund’. As expected, therefore, the origin of ‘God’, Old English of Germanic origin similar to the Dutch ‘god’ and German ‘Gott’.
Even the other sparse explanations found on the web on the origin of the lemma ‘dog’ are unconvincing. I bet your English teacher does not know for sure.

[late OE. docga (once in a gloss); previous history and origin unknown. (The generic name in OE., as in the Teutonic langs. generally, was hund: see HOUND.) So far as the evidence goes, the word appears first in English, as the name of a powerful breed or race of dogs, with which the name was introduced into the continental languages, usually, in early instances, with the attribute ‘English’. Thus mod.Du. dog, late 16th c. dogge (‘een dogghe, vn gros matin d’Engleterre, canis anglicus’, Plantijn Thesaur. 1573), Ger. dogge, in 16-17th c. dock, docke, dogg (‘englische Dock’, Onomast. 1582, ‘eine englische Docke’, 1653), LG. dogge, Da. dogge, Sw. dogg; F. dogue (‘le genereux dogue anglais’, Du Bellay 15..), It., Sp., Pg. dogo, Pg. also dogue; in all the languages applied to some variety or race of dog.]

Why is “dog” one of the great mysteries of the English language?