The genus name, Achillea is derived from Achilles, the hero of Homer's Illiad in Greek mythology. He had been a student of Chiron, a centaur that was apparently renowned for his knowledge of medicinal herbs. Or it may have been that Achilles needed to use yarrow often on his wounded soldiers as they fought thier bloody battles. The list of medicinal uses for this plant is very long just locally; from the Ditidaht and Makah using it as childbirth medication, the Saanich using it in a poultice, the Nuu-chah-nulth for colds and coughs and the Cowichan as a blood-purifier, just to name a few.
The species portion of the name, millefolium, means "thousand leaves" , referring to the finally segmented fern like leaves. You count and see if there are a thousand and let us know.
Addendum: I thought I was done, but just found this lovely gem from http://www.arkive.org/yarrow/achillea-millefolium/.
- "In East Anglia, this property of the plant was employed in order to divine future love; a leaf was placed inside the nose and the following rhyme was recited: ‘Yarroway, yarroway, bear a white blow, if my love love me, my nose will bleed now’"