The eastern region of Númenor (UT/165), a combination of the prefix orro- of orró- “east” and the plural of the suffix -sta “land”.
Quenya
Orrostar
eastlands
orrostar
place name. Eastlands
Elements
Word Gloss orró- “uprising, sunrise, east” -sta “land, part, part; [ᴹQ.] close grouping, land”
Orrostar place-name, the "Eastlands" of Númenor (UT:165)