River marking the southern border of Eriador (LotR/1039), translated “Border-river” (UT/261, VT42/7). This name is combination of glan(n) “border” and duin “river” (VT42/7-8).
Sindarin
Glanduin
noun. border river
glanduin
place name. Border-river
Elements
Word Gloss glan(n) “boundary” duin “(large) river, (large) river; [N.] water”
gland (“boundary”) + duin (“long and large river”) David Salo: “dh and mh were liable to revert to d and m when they came to follow a nasal after syncope” TolkLang message 19.31.