Co-Pilot is just a military term for the same function as an FO.
I would not call my First Officer anything other than that however out of respect for his, or her hard earned position and that is their position in my airline.
The Astronauts at NASA were so into their titles they insisted that the left seater on the Space Shuttle was ranked as 'Commander' with the right seater as 'Pilot'
There could be no 'Co Pilot's' in that rarefied world, funny but understandable considering the requirements to get to that seat !