hardened against persuasion, the sentiment of pity, etc.; stubborn, obstinate, unyielding, inflexible, relentless, hard-hearted ...1672 obs. rare
from Latin obdurat- (hardened, stubborn) + -ious
FIRST DOCUMENTED USE
1672 - see EXAMPLE below
"... I had been guilty of an obduratious self-saving, and perfidious silence...."
From: The Church told of Mr. Ed. Bagshaw's Scandals
- Richard Baxter