Green’s Dictionary of Slang

an excl. of asseveration; I’ll be damned! under damn v.

1701		Cibber Love Makes a Man III i: You may look as terrible as you please, I must banter you, flux me.
1772		Bridges Burlesque Homer (3rd edn) 238: But flux me, tho’ you’ll think it odd, / I’ve a great notion you’re a God.
1797		Bridges Burlesque Homer (4th edn) I 331: Flux me if I will pass your door!