The Collaborative International Dictionary of English v.0.48:

Gamp \Gamp\ (g[a^]mp), n.
   A large umbrella; -- said to allude to Mrs. Gamp's umbrella,
   in Dickens's "Martin Chuzzlewit."
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

gamp
    n 1: colloquial terms for an umbrella [syn: gamp, brolly]