What does "lather" mean?

Lather: Foam from profuse sweating, as of a horse.

Additional senses

  1. 2.To spread over with lather; as, to lather the face.
  2. 3.To form lather, or a froth like lather; to accumulate foam from profuse sweating, as a horse.
  3. 4.To beat severely with a thong, strap, or the like; to flog. [Low]

Sources