IPA: /ˈmɪd.lɪŋ.li/
KK: /ˈmɪd.lɪŋ.li/
In a way that is average or moderate, neither very good nor very bad.
The movie was middlingly entertaining, not great but not terrible either.
Middlingly → It is formed from "middle" (from Old English "midde", meaning the center or midpoint) and "-ling" (a suffix used to form adjectives or adverbs indicating a manner or quality) and "-ly" (a suffix used to form adverbs). The word "middlingly" means in a manner that is average or moderate, reflecting a quality of being in the middle.
Think of being in the 'middle' — 'middlingly' describes doing something in an average or moderate way.