Source : Webster's Revised Unabridged Dictionary (1913)

Plimsoll's mark \Plim"soll's mark`\ (Naut.)
   A mark conspicuously painted on the port side of all British
   sea-going merchant vessels, to indicate the limit of
   submergence allowed by law; -- so called from Samuel
   Plimsoll, by whose efforts the act of Parliament to prevent
   overloading was procured.