êl (“star”) + ros (“foam, spray”) In PM:369, the last element is said to be rôs from the Bëorian tongue.
Elrosnoun. star foam, spray
elrosmasculine name. Star Foam, Star-foam, Star-glitter
Brother of Elrond who chose life as a mortal Man and became the first king of Númenor (LotR/1034-5). His name is translated “Star-foam” (Let/448, PM/349), a combination of êl “star” and ross¹ “foam” (SA/ros, PM/368-9).
Conceptual Development: The name N. Elros first appeared in the Silmarillion drafts from the 1930s (SM/155 note #9, LR/216) and was mentioned in Lord of the Rings drafts from the 1940s (WR/158). In a few places Tolkien considered alternate etymologies for this name: in a letter to Rhona Beare from 1958, he suggested the initial element might be archaic †Ell “Elf” (Let/281), and in an essay on “The Problem of Ros” from 1968, he considered but ultimately rejected the possibility that the final element was a Bëorian word rôs “foam”, instead of Sindarin. Both of these seem to have been transient ideas.
Elros is a Sindarin name meaning "Elf of the spray", based on a tale from his early childhood when the Sons of Fëanor abducted the twins until Maedhros found them playing in a forest waterfall. Alternatively, his name could mean "star-foam". The Sindarin elements are el ("star") and ros ("foam"). Elros is the Sindarin form of Quenya Elerossë (pron. [ˌeleˈrosːe]).