Be on the wane phrase
Be becoming gradually smaller, weaker, less important, etc.
I feel like her enthusiasm for the work is on the wane.
We should reduce the number of that product because the demand for it is on the wane.
The rate of violent crime nationwide has been on the wane since that law was passed.
The origin of this phrase is not clear.
Meaning absolutely certain or without a doubt
I am as sure as God made little green apples that Peter's the one for you, you two make a beautiful couple.