Probably influenced by (or perhaps an alteration of) yatter or yatata; perhaps onomatopoeic of blather; or perhaps derived from the Norwegian expression jada, jada which has a similar pronunciation and interpretation. Sometimes popularly attributed to Yiddish, but this is dismissed by etymologists. "Yatter, yatter" is British (specifically Scots) English for "continuous chatter, rambling and persistent talk, nagging". S. R. Crockett, The Men of the Moss-Hags (1895) xxix: "The woman's yatter, yatter easily vexed me." Yadder is a Cumberland word meaning "to talk incessantly; to chatter". Various variant forms appear in the US 1940s–60s; for example, the 1947 American musical Allegro by Oscar Hammerstein II and Richard Rodgers contains a song called “Yatata, Yatata, Yatata,” about cocktail party chatter; see talk page for additional citations. The phrase "yadda yadda" was first popularized by the comedian Lenny Bruce in his standup bit "Father Flotsky's Triumph," the closing track on his 1961 album "Lenny Bruce - American." It appeared on television in 1993 in the sitcom "Frasier" (Season 4 Episode 12) when Niles and Frasier are having a conversation seen from a dog's point of view. The phrase gained renewed popularity in the US in the late 1990s on the television show Seinfeld, where it appears as a catchphrase, initially in Season 8, Episode 19, entitled “The Yada Yada”, originally aired on April 24, 1997, which centers on the phrase (in the duplicative “yada yada” form).

and so forth • and so on • and what have you • and whatnot • blah • blah blah blah