The name means "boundary stream". Mering represents Old English māere, mēre, "boundary". Glanhír comes from Sindarin glan, "mark, boundary" and sîr, "river, stream".
Sindarin
glanhír
place name. Boundary Stream
Elements
Word Gloss glan(n) “boundary” sîr “river, stream”
Sindarin name of the Mering Stream in Rohan, translated “boundary stream” (UT/318). This name is combination of glan(n) “border” and the lenited form of sîr “river”.