oestrum

noun3

1. •
   (no longer used) Estrus.
2. •
   (figurative, no longer used) Something that stings or incites one to frenzy or action; an impulse or passion.

   • […]at Other times flow'd Easy his Unpremeditated Verse, with a certain Impetus and Aestro, as Himself seem'd to Believe. — 1734
3. •
   (no longer used) A biting insect; a gadfly or botfly.

   • In the oestrum or gadfly, the wimble draws out like the pieces of a spy-glass […]. — 1802