An element in the Quenya name for “kingsfoil”: asëa aranion (LotR/864), which was itself sometimes translated as “asea of the Kings” (PE17/49, 100). In Definitive Linguistic Notes (DLN) from 1959 Tolkien derived asea from √ATHA “as name of plant athelas” (PE17/148), and in notes on Words, Phrases and Passages from the Lord of the Rings around the same period it was derived from √ATH “ease, comfort, heal” (PE17/49). In notes from 1969 Tolkien describe it thus:
> Q asea (< aþāya) name (as = “beneficial”?) of a herb — not now known or at least identifiable, useful in healing both (as an infusion of its leaves) in easing the pain and hastening the healing of heavy blows or shocks, and as an invigorating odour in reviving the sick from depression, or even unconsciousness, after wounds or shock (PE22/165).
In these notes Tolkien first derived healing words from √AÞA, but he then introduced a new root √HATHA “treat kindly/make easy, (help to) cure” (PE22/166 note #109). In rough notes following this he gave {asea >>} aþea with the hard-to-read gloss “treating[?]. medically [?use], caring[?]” (PE22/166 note #110). This line of reasoning was left unfinished.
Neo-Quenya: Some of the above implies that asea was itself another name of the plant S. athelas, but for purposes of Neo-Quenya I think it is more useful to assume asea can refer to any kind of healing herb, and that “kingsfoil” is more specifically asëa aranion.
asëa (þ) 1) adj. "beneficial, helpful, kindly" _(so according to a late note where the word is derived from *ATHAYA)_; hence also: 2) asëa (þ) noun,name of the healing plant called in Sindarin athelas(PE17:148), in English (representing Westron) called "kingsfoil", cf. longer Quenya name asëa aranion (þ) "asëa of kings" (LotR3:V ch. 8). Cf. aran.