Green’s Dictionary of Slang

(US) an expression of affirmation.

1905		Tucumcari News (NM) 2 Dec. 2/1: ‘Ain’t it the Truth?’ [...] It is said that a municipal elections costs New York $2,000,000.
1912		Day Book (Chicago) 5 Dec. 24/1: Gee whiz, ain’t it the truth that the surest thing in the show bisness is, you never can tell!
1917		Day Book (Chicago) 8 Jan. 17/2: Ain’t it? When little Benjamin bites his finger nails he gets a rap over his knuckles, but if a baby eats his whole foot the folks thinks its cute.
1920		Labor World (Duluth, NM) 7 Feb. 6/4: Ain’t It the Truth? Just the same in U.S.
1927		K. Gritney Gritty People 35: Nobody but Roxy an’ you in the house to bother ’bout, ain’t it?
1965		(con. 1930s) R. Wright Lawd Today 86: There’s a nasty taste in yo’ mouth, ain’t it?
1967		(con. 1950s) McAleer & Dickson Unit Pride (1981) 239: Ain’t that the truth? Everybody’s got it made but the poor sonofabitchin’ infantryman.
1972		J. Speight ‘The Bird Fancier’ Till Death Us Do Part [TV script] Well that’s yer bloody Labour Government that is, annit?
1988		Eble Campus Sl. Mar. 1: ain’t it the truth – expression of empathy.