indefeasible

We acquired "indefeasible" in the mid-16th century by combining the English prefix in- ("not") with "defeasible," a word borrowed a century earlier from Anglo-French. "Defeasible" itself can be traced to an Old French verb meaning "to undo" or "to destroy." It's no surprise, then, that something indefeasible is essentially "un-undoable" or "indestructible." Another member of this family of words is feasible, meaning "capable of being done or carried out." Ultimately, all three - "indefeasible," "defeasible," and "feasible" - can be traced back to the Latin verb facere, meaning "to do."