1680s, agent noun from yawn (v.). Meaning "boring thing" is 1942, American English colloquial (yawn (n.) in this sense is attested from 1889).
Entries linking to yawner
yawn v.
c. 1300, yenen, yonen, from Old English ginian, gionian "open the mouth wide, yawn, gape," from Proto-Germanic *gin- (source also of Old English giwian, giowian, giwan "to request," Old Norse gina "to yawn," Dutch geeuwen, Old High German ginen"to be wide open," German gähnen "to yawn"), from PIE root *ghieh- "to yawn, gape, be wide open." Modern spelling is from 16c. Related: Yawned; yawning.