The Collaborative International Dictionary of English v.0.48:

Thumb \Thumb\, n. [OE. thombe, thoumbe, [thorn]ume, AS.
   [thorn][=u]ma; akin to OFries. th[=u]ma, D. duim, G. daumen,
   OHG. d[=u]mo, Icel. [thorn]umall, Dan. tommelfinger, Sw.
   tumme, and perhaps to L. tumere to swell. [root]56. Cf.
   Thimble, Tumid.]
   The short, thick first digit of the human hand, differing
   from the other fingers in having but two phalanges; the
   pollex. See Pollex.
   [1913 Webster]

         Upon his thumb he had of gold a ring.    --Chaucer.
   [1913 Webster]

   Thumb band, a twist of anything as thick as the thumb.
      --Mortimer.

   Thumb blue, indigo in the form of small balls or lumps,
      used by washerwomen to blue linen, and the like.

   Thumb latch, a door latch having a lever formed to be
      pressed by the thumb.

   Thumb mark.
   (a) The mark left by the impression of a thumb, as on the
       leaves of a book. --Longfellow.
   (b) The dark spot over each foot in finely bred black and tan
       terriers.

   Thumb nut, a nut for a screw, having wings to grasp between
      the thumb and fingers in turning it; also, a nut with a
      knurled rim for the same perpose.

   Thumb ring, a ring worn on the thumb. --Shak.

   Thumb stall.
   (a) A kind of thimble or ferrule of iron, or leather, for
       protecting the thumb in making sails, and in other work.
   (b) (Mil.) A buckskin cushion worn on the thumb, and used to
       close the vent of a cannon while it is sponged, or
       loaded.

   Under one's thumb, completely under one's power or
      influence; in a condition of subservience. [Colloq.]
      [1913 Webster]