A juggernaut is something (such as a force, campaign, or movement) that is extremely large and powerful and cannot be stopped.
In the early 14th century, Franciscan
missionary Friar Odoric brought to Europe the story of an enormous carriage
that carried an image of the Hindu god Vishnu (whose title was Jagannāth,
literally, "lord of the world") through the streets of India in
religious processions. Odoric reported that some worshippers deliberately
allowed themselves to be crushed beneath the vehicle's wheels as a sacrifice to
Vishnu.
That story was likely an exaggeration
or misinterpretation of actual events, but it spread throughout Europe. The
tale caught the imagination of English listeners, and they began using
juggernaut to refer to any massive vehicle (such as a steam locomotive) and to
any other enormous entity with powerful crushing capabilities.
While the word is still used sometimes
in British English to refer to a very large, heavy truck (also called a
"juggernaut lorry"), juggernaut is more commonly used figuratively
for a relentless force, entity, campaign, or movement, as in "a
political/economic/cultural juggernaut."