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:
namas te gaura vānī šrī mūrtaye dīna tārine
rūpānuga viruddhāpasiddhānta dhvānta hārine
gaurāvirbhāva bhūmes tvam nirdeštā saj jana priyah
vaišnava sārvabhaumah šrī jagannāthāya te namah
in both verses "namah te" and "te namah" mean the same thing, "I bow down before you".
Om Namah shivaya indicates positive energy. If any body chants the reverse that means "Yavashih Mana Mo" its a negative energy. Generally used in voodoo or Black magic. For any query watch Vikramaditya YouTube channel on black magic.