Ras is a Sindarin word for "horn" (cf. Caradhras); Morthil likely contains the element mor, "dark, black".
Sindarin
ras morthil
place name. Andrast
Elements
Word Gloss ras(t) “cape, shore” morn “black, dark; night” till “point, spike, (sharp) horn, tine, ending”
Ras Morthil
Ras Morthil
Apparently another name for Andrast (UT/175, 214 note #6). This name seems to be a combination of ras(t) “cape”, morn “black” and the nasal mutation of till “point” (as suggested by David Salo, GS/389).