Green’s Dictionary of Slang

jacks n.¹

In exclamations

by jacks! (also by Jack! by jackers! by Jakers! Jakers!)

used in mild oaths as a euph. for by Jesus!

c.1785		‘The Connaughtman’s Visit to dublin’ in A. Carpenter Verse in Eng. in 18C Ireland (1998) 390: But she svore by her jakers she vood have her due.
c.1790		‘The Connaughtman’s Visit to Dublin’ Luke Caffrey’s Gost [as previous].
1837		‘The Devil and Johnny Dixon’ in Bentley’s Misc. Mar. 254: ‘Oh, be Jakers!’ says the priest.
1842		S. Lover Handy Andy 248: By Jakers, I’m sorry I towld you.
1843		Gleaner (Manchester, NH) 15 July n.p.: By jackers, what a quaking the mammoth bones are making.
1850		Leeds Intelligencer 9 Feb. 7/6: The revellers were struck with astonishment on seeing a policeman come out of the sea. The captain exclaimed, ‘by jakers, here’s Neptune’.
1899		A.C. Gunter M.S. Bradford Special 89: By Jack!
1914		J.C. Ruppenthal ‘A Word-List From Kansas’ in DN IV:ii 104: by-word, n. A mild oath, as [...] ‘by jacks’.
1920–23		J. Wells in Hunter Trail Drivers of Texas (1963) I 165: Cook says, ‘By Jacks, when it begins to thunder you fill this wagon full of six-shooters, but when the Indians are around the guns are all gone.’.
1922		Joyce Ulysses 310: O Jakers, Jenny, says Joe, how short your shirt is!
1943		W. Guthrie Bound for Glory (1969) 89: If she’s got a motor in ’er, by jacks, I wanta see it!