labiumEnglishOxford Dictionary of Englishlabium /ˈleɪbiəm / ▸ noun (plural labia /ˈleɪbɪə/) 1 Entomology a fused mouthpart which forms the floor of the mouth of an insect. 2 Botany the lower lip of the flower of a plant of the mint family. – ORIGIN late 16th century (in the general sense ‘lip, liplike structure’): from Latin, ‘lip’; related to labrum.