The Collaborative International Dictionary of English v.0.48:

fibroin \fi"bro*in\ (f[imac]"br[-o]*[i^]n or
   f[i^]b"r[-o]*[i^]n), n. [L. fibra a fiber.] (Chem.)
   A variety of gelatin; the chief ingredient of raw silk,
   extracted as a white amorphous mass.
   [1913 Webster]