Definition of middlingness in English:
middlingness
noun ˈmɪd(ə)lɪŋnɪsˈmɪdlɪŋnɪs
The state of being middling; mediocrity.
Origin
Mid 19th century; earliest use found in ‘George Eliot’ (1819–1880), novelist (real name Mary Ann Evans). From middling + -ness.