A noun for “blood” appearing as an element in the name Agarwaen (S/210). It’s later etymology is unclear.
Conceptual Development: An earlier iteration of the name was N. {Iarvael >>} N. Iarwath “Blood-stained” from The Etymologies of the 1930s, where the element was N. iâr “blood” from the root ᴹ√YAR of the same meaning (Ety/YAR). Tolkien considered changing the root to ᴹ√YOR and the Noldorin form to iûr (EtyAC/YAR). This seems to be transient idea, since the name Iarwaeth “Bloodstained” appeared in the Grey Annals from the early 1950s (WJ/83) before ultimately being replaced by Agarwaen (WJ/142).
Neo-Sindarin: For purposes of Neo-Sindarin, I’d stick to the better-described sereg for “blood”.
A name adopted by Túrin in Nargothrond to hide his identity (S/210). It is translated “Blood-stained”, and seems to be a compound of agar “blood” and the lenited form waen of gwaen “stained”, but neither of these words are otherwise attested.
Conceptual Development: In The Etymologies this name appeared as N. Iarwath, a compound of N. iâr “blood” and gwath “stain” (Ety/YAR, WAƷ). Another variant, N. iârvael, appeared in some orthography notes from a slightly later period (PE22/67). A third variant form Iarwaeth appeared in Silmarillion revisions from the 1950s-60s, but it was later rejected and changed to Agarwaen as in the published Silmarillion (WJ/83, 256).