28 April 2015
The short answer is that we don’t know for sure where the word dog comes from. Canis may be familiarus, but its name is something of a mystery.
Dexter, a beagle, photo by Lori Cheung, 2006
The word docga does go back to Old English, but it appears only three times in the extant corpus of pre-Conquest writing, once in a gloss, and three times as part of a place name. The genitive plural form docgena glosses the Latin canum, and is used twice in the description of property boundaries in a charter: doggene ford (dog’s ford) and doggene berwe (dog’s hill). The place name doggiþorn (dog-thorn) appears in another charter. In the twelfth century, the surname Dogheafd (Doghead) is recorded, and several other surnames that use dog as an element date to the post-Conquest era.
The use of the word to refer directly to the animal doesn’t appear in any extant writing before the early thirteenth century, when it is used several times in the work Ancrene Riwle (also known as Ancrene Wisse) a guide for anchoresses, women who withdrew from society, even monastic society, living as hermits within a monastery or church. This particular passage is a description of the devil:
His teð beoð attri as of amad dogge. dauið iþe sauter cleopeð him dogge.
(His teeth are venomous as of a mad dog. David in the Psalter calls him dog.)
Ancrene Riwle appears in many manuscripts. This quotation is taken from the manuscript Cambridge, Corpus Christi College 402, which was copied c. 1230. The original work was probably written before 1200.
Cognates of dog appear in a number of European languages, but these are all much later, and it seems that they are all borrowed from English. With one possible exception, it’s not related to any other Germanic words, so it’s origin is a mystery. The exception may be the word pig, which shows a similar form and semantic development. (See pig.) Like dog, the word pig can be traced to Old English, but it’s not used as a standalone word in the extant corpus. It appears once, in a compound: picbred (pigbread or pigfood), and glosses the Latin Glanx glandis (acorn). The Old English root is likely *picg.
There is, however, a plausible suggestion as to why dog is so rare in extant Old English writings. It could have been a hypocoristic term, that is, a term used as a nickname or by small children. And since almost all surviving Old English writing was written down by monks for specific religious or legal purposes, such informal words simply do not appear in the extant corpus.
The more common Old English term for a dog is hund. This word has a common Germanic root—Hund is the modern German word for dog, for example. And the Old English word survives as hound, although today that word has specialized to refer to a hunting dog, especially one that tracks prey by scent. So docga shows a similar semantic development as *picg, which has largely supplanted swin, the more common Old English word.
Sources:
Dictionary of Old English, University of Toronto, 2009, s. v. “docga.”
Dictionary of Old English Web Corpus, University of Toronto, 2009.
Liberman, Anatoly, Word Origins...and How We Know Them. Oxford University Press, 2005.
Oxford English Dictionary Online, third edition, s. v. “dog, n.1” (November 2010), “pig, n.1” (March 2006).