Green’s Dictionary of Slang

an important or the most important person.

1840		Times-Picayune (New Orleans, LA) 21 June 2/2: It was fairly to be inferred that Stocker was the ‘big dog of the tanyard’ and able to whip either of the others with ease.
1845		J.J. Hooper Adventures of Captain Simon Suggs (1851) 126: The reverend gentleman [...] who Simon was convinced was ‘the big dog of the tanyard’.
1847		in N.E. Eliason Tarheel Talk (1956) 260: Dick [...] pretends to be the biggest dogg [sic] in the meat house.
1859		Spirit of Democracy (Woodsfield, OH) 6 July 2/4: Whoop! I’m just nat’ally spilin’ for a fight! [...] I’m the big dog of the tanyard — the gray wolf of the prairie, so I am!
1870		Louisiana Democrat (Alexandria, LA) 12 jan. 2/2: The great work [...] was to settle whether Warmouth or Wickliffe is to be the ‘big dog’ of the Radical tan-yard.
1883		Memphis Dly Appeal (TN) 9 June 2/3: He has been the ‘big dog’ in the indiana tanyard so long that he will permit no rival in his domain.
1889		Lincoln Co. Leader (White Oaks, NM) 28 Sept. 1/5: We all know there is a Kansas City in [...] Missouri, which is the ‘big dog in the tanyard’.
1898		Lafayette Gaz. (LA) 30 July 1/5: When the news of the great victory of [Teddy] Roosevelt’s Rough Riders was flashed over the wires, Teddy [a puppy] was looked upon as the ‘big dog of the tanyard’.
1911		Arizona Republican (Phoenix, AZ) 31 May 10/3: John, who has been called the big dog of our tanyard has been a popular member of our class.