from The Century Dictionary.
- noun Open space; hence, unrestricted opportunity.
Sorry, no etymologies found.
This man, when he saw Darius intended to fall upon the enemy in the passes and defiles, advised him earnestly to keep where he was, in the open and extensive plains, it being the advantage of a numerous army to have field-room enough when it engages with a lesser force.
The Lives of the Noble Grecians and Romans Plutarch 2003
Which hearts, for want of field-room, cannot bear.