The Collaborative International Dictionary of English v.0.48:

Thummim \Thum"mim\, n. pl. [Heb., pl. of th[=o]m perfection.]
   A mysterious part or decoration of the breastplate of the
   Jewish high priest. See the note under Urim.
   [1913 Webster]

Easton's 1897 Bible Dictionary:

Thummim
   perfection (LXX., "truth;" Vulg., "veritas"), Ex. 28:30; Deut.
   33:8; Judg. 1:1; 20:18; 1 Sam. 14:3,18; 23:9; 2 Sam. 21:1. What
   the "Urim and Thummim" were cannot be determined with any
   certainty. All we certainly know is that they were a certain
   divinely-given means by which God imparted, through the high
   priest, direction and counsel to Israel when these were needed.
   The method by which this was done can be only a matter of mere
   conjecture. They were apparently material objects, quite
   distinct from the breastplate, but something added to it after
   all the stones had been set in it, something in addition to the
   breastplate and its jewels. They may have been, as some suppose,
   two small images, like the teraphim (comp. Judg. 17:5; 18:14,
   17, 20; Hos. 3:4), which were kept in the bag of the
   breastplate, by which, in some unknown way, the high priest
   could give forth his divinely imparted decision when consulted.
   They were probably lost at the destruction of the temple by
   Nebuchadnezzar. They were never seen after the return from
   captivity.


Hitchcock's Bible Names Dictionary (late 1800's):

Thummim, perfection; truth