undertoad
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.
Examples
2000
Masekela remains bitter about the legacy of apartheid: "When I came back South Africa was supposed to be some kind of miracle.

But it isn't like that. There is still an undertoad (sic) of fear and selfishness here.
—Nicola Graydon, “Masekela comes up trumps,” The Evening Standard, November 30, 2000
1993
There was a desire to believe that things could change with the guard. Yet there was also an undercurrent — what John Irving might call an "undertoad" — of doubt.
—Ellen Goodman, “Caution amid the optimism,” The Boston Globe, January 24, 1993
1981 (earliest)
Interpret the look. Is it resentment? No, we see that look daily in the grocery markets. Then what is this jumble of contorted, facial lines surrounding the alert, sharpened focus in the eyes? What feeling is concealed behind the taut expression?

It's fear caused by anticipation of the unknown. Garp's "undertoad."
—Richard Sigal, “Inflation as a deflator of the masculine role,” The New York Times, April 19, 1981
Notes
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.
—John Irving, The World According to Garp, E. P. Dutton, January 01, 1978
Filed Under
anger and anxiety
Posted: July 21, 1997