Mercian. adj. 1. (Languages) of or relating to Mercia or the dialect spoken there. n. 2. (Languages) the dialect of Old and Middle English spoken in the Midlands of England south of the River Humber. 3. (Peoples) a native or inhabitant of Mercia. See also Anglian, Northumbrian. (ˈmɜr ʃi ən, -ʃən). n. adj.