ˈunderfall,n. [under-1 5 b.] A foot-hill slope. 1857Smith's Dict. Grk. & Rom.Geog. II. 1274/2 The underfalls of the Alps that are thrust forward towards the plain.1883Standard 8 May 4/8 The last ‘underfalls’ of the..plateau terminate to the North of Tonquin.