I'll be hanged

1

I am very surprised, as in, Well, I'll be hanged; there's Susan.

2

I'll be hanged if I. Under no circumstances will I, as in I'll be hanged if I let you do that. Both of these hyperbolic colloquial usages allude to being executed by hanging.

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.