Definition of "underlip"

He bit his under-lip as though he were somewhat unwilling, but arose.

1866, Charles Dickens, The Signal-Man‎

His eyes were closed, his mouth opened. His breathing, irregular for three, then four, then five breaths, returned to its normal, soundless rhythm. Stubbled overlip and wet underlip moved about some final, silent word: […]

2014, Samuel R. Delany, Tales of Nevèrÿon