The Collaborative International Dictionary of English v.0.48:

Petersham \Pe"ter*sham\ (p[=e]"t[~e]r*sh[a^]m), n. [Named after
   Lord Petersham.]
   A rough, knotted woolen cloth, used chiefly for men's
   overcoats; also, a coat of that material.
   [1913 Webster]