An unstoppable crushing force, from Jagannath, Lord of the Universe, the deity of Puri whose colossal chariot once a year rolls through the city and cannot be stopped, steered, or controlled.
Documented
The chariot
Documented: Jagannath, Sanskrit jagat, world, plus natha, lord, is the deity of the great temple at Puri, Odisha. The annual Rath Yatra moves the wooden images of Jagannath, Balabhadra, and Subhadra on towering chariots pulled by thousands. The English loanword for the chariot itself appears in the seventeenth century; the figurative crushing force is mid-nineteenth century.
Assessment: Documented: the Sanskrit compound, the Rath Yatra record, the OED's dating. The chariots are still built new every year.
Stories
The wheel martyrs
The story Europe could not put down, from Odoric's fourteenth-century account and Mandeville's retelling: devotees dying deliberately beneath the wheels as sacrifice. It was at minimum an exaggeration, by 1825 observers reported the practice had long ceased, but the image, unstoppable wheels plus voluntary death, is exactly the compound the modern word carries: momentum plus worship plus collateral.
Assessment: Stories: the deaths were rare; the telling was industrial. The word preserves the telling.
Interesting claims
The secularization
The drift: lowercase juggernaut shed the god entirely, steam locomotives, then corporations, then communism, then a Marvel villain, while Puri kept rolling the actual chariot every year, indifferent. The word is a deity who emigrated and lost his temple papers.
Assessment: Interesting: English took the wheel and left the Lord. The archive files it under names that outran their gods.
Sources and further reading
Oxford English Dictionary, juggernaut: from Jagannath, Sanskrit jagat, world, plus natha, lord; chariot sense seventeenth century, figurative sense mid-nineteenth
Friar Odoric's fourteenth-century account of the Rath Yatra (the wheel-martyr story's European origin)
The Puri Rath Yatra record, annual chariot construction and procession (documented continuity)
By 1825: observers report the practice of self-sacrifice had long ceased (the exaggeration's documented correction)
Worth asking
How did a god of mercy become the English word for a crushing machine?