Elrohir,
Elladan: these names, given to his sons by Elrond,
refer to the fact that they were ‘half-elven’ (III 314): they had mortal as
well as Elvish ancestors on
both
sides;
Tuor on their father's side, Beren on their mother's. Both signify
elf+man.
Elrohir might be
translated ‘Elf-knight’;
rohir being
a later form (III 391) of
rochir‘horse-lord’ from
roch‘horse’
+ hir‘master’: Prim. Elvish
rokkō and
khēr or
kherū: High-elven
rocco,
hēr (
hěru).
Elladan might be
translated 'Elf-Númenórean'.
Adan (pl.
Edain) was the Sindarin form of the name given to the ‘fathers of
men’, the members of the Three Houses of Elf-friends, whose survivors
afterwards became the Númenóreans, or
Dún-edain.The Letters of J.R.R. Tolkien, letter no. 211, to Rhona Beare, October 1958 (underlining emphasis added)