E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Botanically called Lysimachia, a Greek compound meaning the same thing. The author of
Flora Domestica tells us that the Romans put these flowers under the yokes of oxen to keep them from quarrelling with each other; for (says he) the plant keeps off flies and gnats and thus relieves horses and oxen from a great source of irritation. Similarly in Collins Faithful Shepherdess, we read
Yellow Lysimachus, to give sweet rest,
To the faint shepherd, killing, where it comes,
All busy gnats, and every fly that hums.
(Pliny refers the name to one of Alexanders generals, said to have discovered its virtues.)