The Collaborative International Dictionary of English v.0.48:

Thatch \Thatch\, v. t. [imp. & p. p. Thatched; p. pr. & vb. n.
   Thatching.] [From Thatch, n.: cf. OE. thecchen, AS.
   ?eccean to cover.]
   To cover with, or with a roof of, straw, reeds, or some
   similar substance; as, to thatch a roof, a stable, or a stack
   of grain.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thatch \Thatch\, n. [OE. thak, AS. [thorn][ae]c a roof; akin to
   [thorn]eccean to cover, D. dak a roof, dekken to cover, G.
   dach a roof, decken 8cover, Icel. [thorn]ak a roof, Sw. tak,
   Dan. tag, Lith. st[=o]gas, Ir. teagh a house, Gael. teach,
   tigh, W. ty, L. tegere to cover, toga a toga, Gr. ?, ?, a
   roof, ? to cover, Skr. sthag. Cf. Deck, Integument,
   Tile, Toga.]
   1. Straw, rushes, or the like, used for making or covering
      the roofs of buildings, or of stacks of hay or grain.
      [1913 Webster]

   2. (Bot.) A name in the West Indies for several kinds of
      palm, the leaves of which are used for thatching.
      [1913 Webster]

   Thatch sparrow, the house sparrow. [Prov. Eng.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

thatch
    n 1: hair resembling thatched roofing material
    2: plant stalks used as roofing material
    3: an English pirate who operated in the Caribbean and off the
       Atlantic coast of North America (died in 1718) [syn: Teach,
       Edward Teach, Thatch, Edward Thatch, Blackbeard]
    4: a house roof made with a plant material (as straw) [syn:
       thatch, thatched roof]
    v 1: cover with thatch; "thatch the roofs"

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "thatch":
   board, brick, clapboard, crop, face, fleece, glass, glaze, head,
   head of hair, lath, locks, mane, mat, mop, paper, plank, revet,
   shag, shake, sheathe, shingle, shock, slate, stone, tile, tresses,
   veneer, wall in, wall up, wallpaper, weatherboard