哞 is an onomatopoeia that represents the sound a cow makes — equivalent to 'moo' in English. It's used in written language, children's stories, and descriptions of farm scenes. You'll see it either standing alone as a sound effect or attached to the verb 叫 (to call) as 哞哞地叫, but this entry is for the single-character onomatopoeia itself. It's not a common word in daily conversation but appears in literature and animal-related contexts.