(n.) someone who believes something utterly unbelievable
A Sebastianist is someone who blindly believes in something utterly unbelievable:
...and there’s too good a story behind this one to leave it unexplained.
The Sebastian in question here is Dom Sebastião de Portugal, or King Sebastian I, who ascended to the Portuguese throne in June 1557 at the age of just 3. Despite his youth, Sebastian’s reign was for the most part successful, and saw Portugal flourish both domestically and internationally with the growth of its colonies in Africa and India.
But in 1578, the now 24-year-old King Sebastian embarked on a risky crusade against Morocco. Ignoring the best counsel of his advisors, Sebastian headed deep into the north African desert to wage war, and on 4 August is presumed to have either been killed in battle at Alcácer Qibir, or else captured and executed soon afterward.
When news broke back in Portugal of the young king’s untimely death, many of the Portuguese people found the loss difficult to accept. As a result, rumours quickly began to emerge that Sebastian had in fact fled the battlefield early in Morocco, or else had escaped captivity, and would one day return to rule Portugal once more.
As the years—and eventually decades—passed by, the legend of the lost king nevertheless endured. Imposters and pretenders to the throne claiming to be the true Sebastian came and went, and belief that the king was indeed still alive persisted—to the extent that when John IV was crowned king in 1640, he had to agree to surrender the throne should Sebastian ever return (despite the fact that, by then, Sebastian would have been well in his 80s).
By the eighteenth century, now more than a hundred years after his presumed death, the belief of Sebastianism had become a pseudo-mystical cult, whose members believed that the legendary king would rise Messiah-like from the dead and return Portugal to its former glory. Despite the cult’s increasingly impossible expectations, its numbers continued to grow—notably, after a renowned proponent of Sebastianism supposedly predicted the powerful earthquake that struck Lisbon in 1755.
Today, the cult of Sebastianism has understandably largely been abandoned. But the term Sebastianist nevertheless remains in use in a broader, more allusive sense to describe anyone who blindly and steadfastly believes something that is truly unbelievable or impossible.