What does "barnacle" mean?

Barnacle: pl. (Far.) An instrument for pinching a horse's nose, and thus restraining him. Note: [Formerly used in the sing.] The barnacles . . . give pain almost equal to that of the switch. Youatt.

Additional senses

  1. 2.pl. Spectacles; -- so called from their resemblance to the barnacles used by farriers. [Cant, Eng.] Dickens.

Sources