(as) sure as a gun American British informal
This expression has been replaced by the phrase "sure as shooting".
With absolute certainty; decisively
The sky is gray, it is sure as a gun raining torrentially.
Jenni is sure as a gun joining the concert tonight since her idol will perform there.
Being recruited in the marketing company she adored, as sure as a gun, she will work there in the long run.
Meaning absolutely certain or without a doubt
To be trustworthy, secure or firm.
Another way to say "absolutely", "definitely" or "for sure"
That's absolutely right.
Totally certain.
It is impossible for a person to change his or her innate or fundamental nature.
He said he would quit alcohol, but he still can't desist from drinking. Once an alcoholic, always an alcoholic.