Green’s Dictionary of Slang

gigs n.¹

In exclamations

by gigs!

a mild euph. for by Jesus!

1575		‘Mr. S’ Gammer Gurton’s Needle in Whitworth (1997) IV i: Chad a foul turn now of late; chill tell it you, by gigs!
1822		R. Nares Gloss. (1888) I 361: By gigs. A corrupt cant oath, perhaps still further depraved from by gis.