Definition of water pimpernel in US English:

water pimpernel

noun

  • 1A brooklime or water speedwell (genus Veronica) (now rare).

  • 2(Often with distinguishing word) any plant of the genus Samolus (family Primulaceae), typically having rosettes of leaves and racemes of small white or pink flowers; also called brookweed.

Origin

Late 16th century; earliest use found in John Banister (1532/3–?1610), surgeon.