The only free and comprehensive online etymological dictionary of the Spanish language
llano (Adjective) "flat"

11th cent. From Latin planus 'id.' From Proto-Italic *plāno- 'id.' From Proto-Indo-European *pleh2-nó- "flattened."

The origin of the surnames de Llano, Llan, Llana, Llanas and Llanos. Further the origin of Llanes, a town in Oviedo.

Romance: Western Vulgar Latin: Asturian llanu, Portuguese chão, Catalan pla, French plan, Italian: piano; Eastern Vulgar Latin: Romanian plan

Indo-European: Balto-Slavic: Old Prussian plonis "threshing floor," Lithuanian plónas "thin," Latvian plãns 'id.;' Anatolian: Hittite palḫi- "wide," Cuneiform Luwian palḫai̯a- 'id.,'