1833, "one who embodies the person or character of another;" 1840 as "one who infuses (something) with a personality;" 1842 as "dramatic actor, one who plays a part on stage," from impersonate with Latinate agent noun suffix. Meaning "one who imitates the manners and speech of another" for entertainment (by 1921) perhaps grew from older theatrical use of female impersonator (1876), male impersonator (1874), both once popular stage acts; the first example of the latter was perhaps Miss Ella Wesner, who had a vogue c. 1870: In Britain, blackface performers were called negro impersonators (1906). As a fem. formation, impersonatrix, as if from Latin, is from 1847; impersonatress, as if from French, is from 1881.
Her [Wesner's] impersonation were a genuine surprise and her success was so pronounced that in a short period a host of imitators made their appearance. Her most successful rivals were Bessie Bonehill, Millie Hilton and Vesta Tilley, all of London. [M.B Leavitt, "Fifty Years in Theatrical Management," New York, 1912]
There is no member of a minstrel company who gets a better salary than a good female impersonator, the line being considered a very delicate one, requiring a high style of art in its way to judge where fun stops and bad taste begins, with decision enough on the part of the performer to stop at the stopping place. ["The Ancestry of Brudder Bones," Harper's New Monthly Magazine, April 1879]
The most fascinating performer I knew in those days was a dame named Metcalfe who was a female female impersonator. To maintain the illusion and keep her job, she had to be a male impersonator when she wasn't on. Onstage she wore a wig, which she would remove at the finish, revealing her mannish haircut. "Fooled you!" she would boom at the audience in her husky baritone. Then she would stride off to her dressing room and change back into men's clothes. She fooled every audience she played to, and most of the managers she worked for, but her secret was hard to keep from the rest of the company. [Harpo Marx, "Harpo Speaks"]
"quadruped of the genus Canis," Old English docga, a late, rare word, used in at least one Middle English source in reference specifically to a powerful breed of canine; other early Middle English uses tend to be depreciatory or abusive. Its origin remains one of the great mysteries of English etymology.
The word forced out Old English hund (the general Germanic and Indo-European word, from root from PIE root *kwon-) by 16c. and subsequently was picked up in many continental languages (French dogue (16c.), Danish dogge, German Dogge (16c.). The common Spanish word for "dog," perro, also is a mystery word of unknown origin, perhaps from Iberian. A group of Slavic "dog" words (Old Church Slavonic pisu, Polish pies, Serbo-Croatian pas) likewise is of unknown origin.
In reference to persons, by c. 1200 in abuse or contempt as "a mean, worthless fellow, currish, sneaking scoundrel." Playfully abusive sense of "rakish man," especially if young, "a sport, a gallant" is from 1610s. Slang meaning "ugly woman" is from 1930s; that of "sexually aggressive man" is from 1950s.
Many expressions — a dog's life (c. 1600), go to the dogs (1610s), dog-cheap (1520s), etc. — reflect the earlier hard use of the animals as hunting accessories, not pets. In ancient times, "the dog" was the worst throw in dice (attested in Greek, Latin, and Sanskrit, where the word for "the lucky player" was literally "the dog-killer"), which plausibly explains the Greek word for "danger," kindynos, which appears to be "play the dog" (but Beekes is against this).
Notwithstanding, as a dog hath a day, so may I perchance have time to declare it in deeds. [Princess Elizabeth, 1550]
Meaning "something poor or mediocre, a failure" is by 1936 in U.S. slang. From late 14c. as the name for a heavy metal clamp of some kind. Dog's age "a long time" is by 1836. Adjectival phrase dog-eat-dog "ruthlessly competitive" is by 1850s. Phrase put on the dog "get dressed up" (1934) may be from comparison of dog collars to the stiff stand-up shirt collars that in the 1890s were the height of male fashion (and were known as dog-collars from at least 1883).
And Caesar's spirit, ranging for revenge,
With Ate by his side come hot from Hell,
Shall in these confines with a monarch's voice
Cry Havoc! and let slip the dogs of war;
[Shakespeare, "Julius Caesar"]