In today’s feed, this One Big Happy from 3/7:
The linguistic point: Ruthie’s mother’s “Ain’t it the truth?” — ain’t in the speech of someone who almost surely isn’t otherwise a user of this word. Instead, she’s playfully quoting a very widespread non-standardism, much as if she’d said “C’est vrai!” or “Veritable!”, in French, in the middle of an English conversation, conveying the equivalent of informal “That’s for sure!” or “You said it!”
The McGraw-Hill Dictionary of American Idioms and Phrasal Verbs (2002) has an entry for “Ain’t it the truth” as a conventionalized expression, both in non-standard varieties and as an importation into informal standard speech:
Rur. or Jocular That is true.; Isn’t that true? (Used to agree with a statement someone has made.) Jane: I swear, life can be a trial sometimes. Bill: Yes, Lordy. Ain’t it the truth?