Trial by fire

any insight on the phrase "trial by fire" would be appreciated.

Dictionary.com says:

trial by fire -- A test of one's abilities, especially the ability to perform well under pressure.

I have had no luck on finding the phrase in any of my references or quote books. I am guessing that it either has to do with "fire" as in gunfire in battle. Or maybe:

Trial by ordeal -- "A test that presents great challenges to patience and courage or to physical and mental endurance..." "Merriam-Webster's Dictionary of Allusions" by Elizabeth Webber and Mike Feinsilber (Merriam-Webster, Springfield, Mass., 1999).