Certain; definite: It's sure as God made little green apples that they'll never get here(1940s+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with sure as shooting
sure as shooting
Most certainly, as in It's going to snow tonight, sure as shooting, or That grizzly is sure as shooting going to make dinner out of us. This idiom has replaced the older sure as a gun, dating from the mid-1600s, a time when guns fired with far less certainty.
[ Second half of 1800s