A bogeyman, or in general, fear and anxiety about the unknown and mortality. From "The World According to Garp".
Jet travel was his undertoad.
(noun) A wrinkly toad that lives in disgusting woman's vagina. If a penis penetrates a vagina inhabited by an undertoad, it is at risk of being bitten off, eaten, and/or savagely mauled. Most undertoad have rows of many razor-sharp teeth, the better to ravage upon your male genitalia and/or objects of pleasure.
Broman: ew, ugh, dude, that chick last night had an undertoad. almost got my dick bit off.
Brosephine: i almost got an undertoad once. turns out i was just giving birth to a small baby shark.