Inexorable
Etymology
From Latin inexorabilis, from in-, "not," and exorare, "to persuade by entreaty" (from ex-, "out," and orare, "to plead, pray"). Its oldest sense is literally "not able to be prayed out of something" — deaf to all pleading. That has broadened into the modern sense of any unstoppable, unswerving force, though the note of being immune to appeal remains.
adjective · /in-EK-suh-ruh-bul/
Impossible to stop, change, or persuade away from its course.
What it names
The relentless quality of a force or process that grinds forward no matter what — and cannot be moved by any plea.
Example
The inexorable rise of the tide left the sandcastle a smooth, anonymous mound by evening.
Related idea
Relentless stresses unceasing, unyielding pressure; inexorable adds the older sense of being impossible to move by any appeal.