Basically for Old English (O.E.) it was docga, which means a powerful dog breed. That is the basic... you can read the rest for more.

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O.E. , a late, rare word used of a powerful breed of canine. It forced out O.E. hund (the general Gmc. and IE word; see canine) by 16c. and subsequently was picked up in many continental languages (cf. Fr. dogue, Dan. dogge), but the origin remains one of the great mysteries of English etymology. Many expressions -- a dog's life (1607), go to the dogs (1619), etc. -- reflect earlier hard use of the animals as hunting accessories, not pampered pets. In ancient times, "the dog" was the worst throw in dice (attested in Gk., L., and Skt., where the word for "the lucky player" was lit. "the dog-killer"), which plausibly explains the Gk. word for "danger," kindynas, which appears to be "play the dog." Slang meaning "ugly woman" is from 1930s; that of "sexually aggressive man" is from 1950s. Dog tag is from 1918. Dogs "feet" is 1913, from rhyming slang dog's meat. To dog-ear a book is from 1659; dog-eared in extended sense of "worn, unkempt" is from 1894. Dogfish is first recorded 1475; dogwood is 1617, earlier dog-tree (1548).
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Source(s):
http://www.etymonline.com/index.php?term=dog


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