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Webster 1913 Edition

Noun.

The lower lip.

underlip ‎(plural underlips)

The lower lip.
- 2014, Samuel R. Delany, Tales of Nevèrÿon (ISBN 1480461733):
  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: […]