Green’s Dictionary of Slang

(Irish) a general excl., a euph. for Jesus! excl. (cf. bejabers! excl.).

1940s–50s		‘Flann O’Brien’ Hair of the Dogma (columns) n.p.: Japers! Yerself? Well, by gor. You’ll have the usual? [BS].
1979		J. Greenaway Tales from the united States 28: ‘Japers!’ Pat roared. ‘Sayze him! sayze him! Sayze him by the tail. Oh, howly harps! [...] Och, japers! japers! He's a fiery sarpint!
1999		D. Healy Sudden Times 84: Japers. He feels his nose very carefully.
2001		G. Coughlan Everyday Eng. and Sl. 🌐 Japers! (exclam): Gosh, really?