Japanese knotweed (Reynoutria japonica, syn. Polygonum cuspidatum)
köken: Uncertain. One explanation is that it is a blend of 痛(いた)み (itami, “pain”) + 取(と)り (tori, “removal”); alluding to the plant's medicinal value. The tori changes to dori as an instance of rendaku (連濁). The kanji is an instance of jukujikun (熟(じゅく)字(じ)訓(くん)).