Green’s Dictionary of Slang

anything or anyone considered tedious, boring and thus productive of yawns.

1885		Bulletin (Sydney) 30 May 10/4: He not only eloped with the wife, but he returned with her the same day to her husband’s house and took formal possession thereof. […] It is now popularly understood that Pentridge prison is now one yawn for that book-agent.
1924		Galsworthy White Monkey 78: I think [...] that Mr. Chalfont is overrated – he’s nothing but a mental yawn.
1930		E. Glyn Flirt & Flapper 12: Flirt: Life was one long excitement. Flapper: [...] it’s one long yawn now.
1944		W. Winchell On Broadway 23 Oct. [synd. col.] ‘The Last Horseman’ is for those who make a hobby out of collecting yawns.
1952		W. Winchell On Broadway 5 Aug. [synd. col.] At Stockbridge (Mass.) they [i.e. an audience] found ‘The Flashing Stream’ [...] a Long Yawn.
1964		Manchester Guardian Weekly 9 Apr. 4: Sir Alec had become a yawn.
1978		R. Price Ladies’ Man (1985) 222: I [...] read an interview with a European ballet star performing at the Lincoln Centre. The guy sounded like a yawn.
1992		E. St Aubyn ‘Never Mind’ Some Hope (1994) 41: All Nicholas’s friends were such wrinklies and some of them were a real yawn.
2000		Indep. Rev. 16 June 7: Really, God, it’s such a yawn.