The corn poppy, Papaver Rhoeas, the odour of which is said to cause headaches; headache.
From Middle English heedwarke, hedewarke, from Old English hēafodwærc (“pain in the head, headache”), equivalent to head + wark. Cognate with Danishhovedvark (“headache”), Swedishhuvudvärk (“headache”), Icelandichöfuðverkur (“headache”).
English Wiktionary. Available under CC-BY-SA license.