Source : Webster's Revised Unabridged Dictionary (1913)

Telharmony \Tel*har"mo*ny\ [?]), at a distant point or points by
   means of alternating currents of electricity controlled by an
   operator who plays on a keyboard. The music is produced by a
   receiving instrument similar or analogous to the telephone,
   but not held to the ear. The pitch corresponds with frequency
   of alternation of current. Telltale \Tell"tale`\, n.
   1. A thing that serves to disclose something or give
      information; a hint or indication.

            It supplies many useful links and telltales.
                                                  --Saintsbury.

   2. (Railroads) An arrangement consisting of long strips, as
      of rope, wire, or leather, hanging from a bar over
      railroad tracks, in such a position as to warn freight
      brakemen of their approach to a low overhead bridge.

Telltale \Tell"tale`\, a.
   Telling tales; babbling. ``The telltale heart.'' --Poe.

Telltale \Tell"tale`\, n.
   1. One who officiously communicates information of the
      private concerns of others; one who tells that which
      prudence should suppress.

   2. (Mus.) A movable piece of ivory, lead, or other material,
      connected with the bellows of an organ, that gives notice,
      by its position, when the wind is exhausted.

   3. (Naut.)
      (a) A mechanical attachment to the steering wheel, which,
          in the absence of a tiller, shows the position of the
          helm.
      (b) A compass in the cabin of a vessel, usually placed
          where the captain can see it at all hours, and thus
          inform himself of the vessel's course.

   4. (Mach.) A machine or contrivance for indicating or
      recording something, particularly for keeping a check upon
      employees, as factory hands, watchmen, drivers, check
      takers, and the like, by revealing to their employers what
      they have done or omitted.

   5. (Zo["o]l.) The tattler. See {Tattler}.

Source : WordNet®

telltale
     adj : disclosing unintentionally; "a telling smile"; "a telltale
           panel of lights"; "a telltale patch of oil on the water
           marked where the boat went down" [syn: {revealing}, {telling},
            {telltale(a)}]
     n : someone who gossips indiscreetly [syn: {tattletale}, {tattler},
          {taleteller}, {talebearer}, {blabbermouth}]