(as) sure as eggs (is eggs) British old-fashioned phrase informal
Used to say that something is certainly going to happen
He'll recover soon, as sure as eggs is eggs.
If you don't study hard, you will fail the exam. Sure as eggs is eggs!
Meaning absolutely certain or without a doubt
Another way to say "absolutely", "definitely" or "for sure"
That's absolutely right.
Totally certain.
With absolute certainty; decisively
It might be changed from the phrase " as sure as x is x".
It was often said by a photographer when he would like small children to look at the camera and smile before he took a photo.
When the photographer said “watch the birdie”, all of us looked at the camera and smiled.