thummim

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]
    

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.
    

Thummim, perfection; truth