The Collaborative International Dictionary
Pennyroyal \Pen`ny*roy"al\, n. [A corruption of OE. puliall royal. OE. puliall is ultimately derived fr. L. puleium, or pulegium regium (so called as being good against fleas), fr. pulex a flea; and royal is a translation of L. regium, in puleium regium.] (Bot.) An aromatic herb ( Mentha Pulegium) of Europe; also, a North American plant ( Hedeoma pulegioides) resembling it in flavor.
Bastard pennyroyal (Bot.) See Blue curls, under Blue.
Wikipedia
Hedeoma pulegioides (American pennyroyal, or American false pennyroyal) is a species of Hedeoma native to eastern North America, from Nova Scotia and southern Ontario west to Minnesota and South Dakota, and south to northern Georgia and Arkansas.
It is a low-growing, strongly aromatic herbaceous annual plant from 15–30 cm tall, with a slender erect much-branched, somewhat hairy and square stem. The leaves are small, thin, and rather narrow, with a strong mintlike odor and pungent taste. The flowers are pale blue, monoecious, produced in small clusters; it flowers from mid to late summer.
Other names are mock pennyroyal, squaw mint, tickweed, stinking balm, mosquito plant, American falsepennyroyal, and American false pennyroyal.
The term "Pennyroyal" (or Pennyrile, from a dialectal pronunciation) is also used to describe a geographic province of western Kentucky, the Pennyroyal Plateau, where H. pulegioides grew in profusion sufficient to lend its name to the whole area.