The Collaborative International Dictionary of English v.0.48:

Barnacle \Bar"na*cle\, n. [Prob. from E. barnacle a kind of
   goose, which was popularly supposed to grow from this
   shellfish; but perh. from LL. bernacula for pernacula, dim.
   of perna ham, sea mussel; cf. Gr. pe`rna ham. Cf. F.
   bernacle, barnacle, E. barnacle a goose; and Ir. bairneach,
   barneach, limpet.] (Zool.)
   Any cirriped crustacean adhering to rocks, floating timber,
   ships, etc., esp.
   (a) the sessile species (genus Balanus and allies), and
   (b) the stalked or goose barnacles (genus Lepas and
       allies). See Cirripedia, and Goose barnacle.
       [1913 Webster]

   Barnacle eater (Zool.), the orange filefish.

   Barnacle scale (Zool.), a bark louse (Ceroplastes
      cirripediformis) of the orange and quince trees in
      Florida. The female scale curiously resembles a sessile
      barnacle in form.
      [1913 Webster]