Someone told a priest that he wants to pray in reverse and the priest told him this:
You are praying like Om Namah Shivay currently. So you need to pray now Shivay Namah Om.
So my question is what is reverse chanting and what does it signify?
Changing the word order in Sanskrit does not reverse the phrase meaning. Meaning stays the same, because word relationships are indicated by inflection.
Word order gets customarily changed in Sanskrit poetry. Example:
in both verses "namah te" and "te namah" mean the same thing, "I bow down before you".