Green’s Dictionary of Slang

figgins n.¹

In exclamations

by my figgins! (also by my fig!) [? SE faith + dimin. sfx -kins]

a general excl. of astonishment or emphasis.

1653		Urquhart (trans.) Gargantua and Pantagruel (1927) I Bk I 15: By my figgins, godmother.
1759–67		Sterne Tristram Shandy (1949) 479: By my fig! said she, swearing, I’ll go no furthur.