Green’s Dictionary of Slang

(US) used in mild oaths, typically by jocks!, i jock!

1878		Donaldson Chief (LA) 23 Mar. 1/2: Well then, by Jimminy Pelt, by dad, by hokey, by the long-armed soon, by jocks.
1889		Dead Bird (Sydney) 17 Aug. 2/1: ‘You wouldn’t,’ replied the rude boy [...] ‘By Jocks, you couldn’t!’.
1899		A.C. Gunter M.S. Bradford Special 203: Jocks! but he’s going to drive missus [...] he is.
1906		J.W. Carr ‘Words from Northwest Arkansas’ in DN III:ii 143: jocks, n. In the mild imprecation, ‘by jocks’.