Is it... "irresistible" force, or "unstoppable" force?

Ultra-Sphix tells Superman, irresistible force.

But Joker tells Batman unstoppable force
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No sorry I don’t equate the terms as synonymous.

And neither does Thesaurus dot com synonym for unstoppable or synonym for irresistible. Neither are found synonymous with the other.

Anyone know the original usage?

How did it morph from one to the other?

Which term do you use?