The story
From the use of the Lancashire dialect terms "yon" and "yonder".
Etymology from Wiktionary, CC BY-SA 4.0.
Yonner
Today it means: An unsophisticated resident of the towns of Oldham, Rochdale or surrounding areas. Now more widely applied to residents of any the satellite towns of Greater Manchester who speak with a broad Lancashire accent. A Lancashire hillbilly or bumpkin. Full definition →