Making sense is overrated. Even trying to be vaguely understood is restrictive.
The Heebyjeeby is, by nature, a solitary, quiet animal, with a small penchant for building nests which it then sells to small mammals or birds, depending on locale, price, amenities, frequency of mobs etc. Almost never does it come into contact with more than one of its own species, and its dealings with other animals it likes it also keeps to a minimum, unless selling them compact and bijou but deceptively spacious living spaces. Then, one fateful day (everyday is fateful in at least one area) a Heeby named Jeeby was prostrate in a jeep, thinking about the good old ways and how to apply for them when an angry squirrel confronted it and accused it of hiding his nuts (they were actually just cheap nuts, and had simply gone). Jeeby quickly vacated the jeep, but ran into another Heeby called Cake, who was running from a stoat who’d fancied a change of nightmare for a day. The two of them set off on an adventure in fleeing, but encountered another pair of heebyjeebies, whose names we shall no longer document, who were running away from things we shall no longer keep track of, because registers are products of the bureaucratic mind, which is like an organic machine, constantly being reducted. One thing led to another and soon there were a whole pack running for all they were worth in a bear market when they actually did run into a bear market, which caused some of the more nervous to start screaming, which set them All off.
And that was where we came in, and how they got their name.