Another name for Arwen (PE17/56), a combination of a variant form of her father’s name Elrond and the feminine suffix -iel. Like its Quenya cognate Elerondiel, it is apparently intended to mean “Daughter of Elrond”. There are several variant forms of both the Sindarin and Quenya names, reflecting Tolkien’s uncertainties on the Elvish words for “daughter”: sell versus iell. See those entries for further discussion.
Sindarin
Elrenniel
noun. 'Daughter of Elrond'
elrenniel
feminine name. Arwen; *(lit.) Elrond-daughter
prop. n. #'Daughter of Elrond'. Elrenniel << Elrendiel << Elrondiel << hel-Elrond << Elrondhel.