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]

The Collaborative International Dictionary of English v.0.48:

Thumb \Thumb\, v. t. [imp. & p. p. Thumbed; p. pr. & vb. n.
   Thumbing.]
   1. To handle awkwardly. --Johnson.
      [1913 Webster]

   2. To play with the thumbs, or with the thumbs and fingers;
      as, to thumb over a tune.
      [1913 Webster]

   3. To soil or wear with the thumb or the fingers; to soil, or
      wear out, by frequent handling; also, to cover with the
      thumb; as, to thumb the touch-hole of a cannon.
      [1913 Webster]

            He gravely informed the enemy that all his cards had
            been thumbed to pieces, and begged them to let him
            have a few more packs.                --Macaulay.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thumb \Thumb\, v. i.
   To play with the thumb or thumbs; to play clumsily; to thrum.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thumb
    n 1: the thick short innermost digit of the forelimb [syn:
         thumb, pollex]
    2: the part of a glove that provides a covering for the thumb
    3: a convex molding having a cross section in the form of a
       quarter of a circle or of an ellipse [syn: ovolo, thumb,
       quarter round]
    v 1: travel by getting free rides from motorists [syn:
         hitchhike, hitch, thumb]
    2: look through a book or other written material; "He thumbed
       through the report"; "She leafed through the volume" [syn:
       flick, flip, thumb, riffle, leaf, riff]
    3: feel or handle with the fingers; "finger the binding of the
       book" [syn: finger, thumb]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "thumb":
   OK, accept, all thumbs, annulary, approve of, awkward,
   be contemptuous of, catch a ride, clumsy, come in contact, contemn,
   dactylion, deride, digit, disapprove of, dismiss, feel, feel of,
   finger, flick, flip through, flout, forefinger, ham-fisted, handle,
   hitch, hitch a ride, hitch rides, hitchhike, index, index finger,
   jeer at, little finger, maladroit, manipulate, medius,
   middle finger, minimus, mock, okay, palm, palpate, paw, pinkie,
   ply, poke at, pollex, prod, rebuff, reject, ring finger, scoff at,
   scorn, tap, thumb through, touch, turn down, twiddle, welcome,
   wield



The Jargon File (version 4.4.7, 29 Dec 2003):

thumb
 n.

    The slider on a window-system scrollbar. So called because moving it allows
    you to browse through the contents of a text window in a way analogous to
    thumbing through a book.


The Free On-line Dictionary of Computing (30 December 2018):

Thumb

    An extension to the Advanced RISC Machine
   architecture, announced on 06 March 1995 by Advanced RISC
   Machines Ltd.  By identifying the critical subset of the ARM
   instruction set and encoding it into 16 bits, ARM has
   succeeded in reducing typical program size by 30-40% from
   ARM's already excellent code density.  Since this Thumb
   instruction set uses less memory for program storage, cost is
   further reduced.

   All Thumb-aware processor cores combine the capability to
   execute both the 32-bit ARM and the 16-bit Thumb instruction
   sets.  Careful design of the Thumb instructions allow them to
   be decompressed into full ARM instructions transparently
   during normal instruction decoding without any performance
   penalty.  This differs from other 32-bit processors, like the
   Intel 486SX, with a 16-bit data bus, which require two
   16-bit memory accesses to execute every 32-bit instruction and
   so halve performance.

   The patented Thumb decompressor has been carefully designed
   with only a small amount of circuitry additional to the
   existing instruction decoder, so chip size and thus cost do
   not significantly increase.  Designers can easily interleave
   fast ARM instructions (for performance critical parts of a
   program) with compact Thumb code to save memory.

   (1995-03-14)


The Free On-line Dictionary of Computing (30 December 2018):

thumb

    The slider or "bubble" on a window system
   scrollbar.  So called because moving it allows you to browse
   through the contents of a text window in a way analogous to
   thumbing through a book.

   [Jargon File]

   (1995-03-14)