n. A form of anxiety, the chief feature of which is an overarching fear of the unknown in general and one's personal mortality in particular.

Example Citation:
"[Timothy Findley's] genuine self-doubt is forever near the surface, threatening to pull him down, and if not destroy him, silence him for good. This is his personal undertoad.
—Quest (as quoted by Anne Soukhanov in Word Watch)

The word undertoad comes from the phrase Under Toad which was coined by John Irving in his book The World According to Garp. In the book, the youngest child, Walt, is constantly being warned to "watch out for the undertow" while playing in the surf, but he mishears the word as Under Toad:
   Garp...realized that all these years Walt had
   been dreading a giant toad, lurking offshore,
   waiting to suck him under and drag him out to
   sea. The terrible Under Toad.