Green’s Dictionary of Slang

yawner n.

[yawn n. + sfx -er]

(US) anything boring, yawn-producing.

[US]Berrey & Van den Bark Amer. Thes. Sl.
[UK]A. Glyn Dragon Variation 42: The game between him and Wheaton, still to be played, should be a real yawner .
[US]Chicago Sun-Times 6 Aug. 76: (heading) Opener likely to be yawner.