E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Flea-bane, the odour being, as it is supposed, hateful to fleas.
This is a real curiosity of blundering derivation. The Latin word is pulcium, the flea destroyer, from pulex, a flea, softened into pulègium, and corrupted into the English-Latin pule-regium. Pule, changed first into puny, then into penny, gives us penny-regium, whence penny-royal. The French call the herb pouliot, from pou (a louse or flea).