telltale

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One who informs on another; a talebearer.
• noun Something that indicates or reveals information; a sign.
• noun Any of various devices that indicate or register information, especially.
• noun A time clock.
• noun Nautical One of the brightly colored lengths of yarn or ribbon attached to the shrouds, stays, or sails of a sailboat, serving to indicate wind direction relative to the boat's motion.
• noun A row of strips hung above a railroad track to warn a passing train of low clearance ahead.
• noun Sports A resonant metal strip, 24 or 30 inches (61 or 76 centimeters) high, across the bottom of the front wall of a racquets or squash court above which the ball must be hit.

from The Century Dictionary.

• noun One who officiously or heedlessly communicates information concerning the private affairs of others; one who tells that which is supposed to be secret or private; a blabber; an informer; a tale-bearer.
• noun An indication or an indicator; that which serves to convey information.
• noun A name given to a variety of instruments or devices, usually automatic, used for counting, indicating, registering, or otherwise giving desired information.
• noun In ornithology, a tattler; a bird of the genus Totanus in a broad sense: as, the greater and lesser telltale, Totanus melanoleucus and T. fiavipes. See tattler, and cut under yellowlegs.
• Disposed to tell or reveal secrets, whether officiously or heedlessly; given to betraying the confidences or revealing the private affairs of others; blabbing: as, telltale people.
• Showing, revealing, or denoting that which is not intended to be known, apparent, or proclaimed: as, telltale tears; telltale blushes.
• That gives warning or intimation of something: as, a telltale pipe attached to a cistern or tank.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who officiously communicates information of the private concerns of others; one who tells that which prudence should suppress.
• noun (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.
• noun A mechanical attachment to the steering wheel, which, in the absence of a tiller, shows the position of the helm.
• noun 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.
• noun (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.
• noun (Zoöl.) The tattler. See Tattler.
• noun A thing that serves to disclose something or give information; a hint or indication.
• noun (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.
• adjective Telling tales; babbling.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One who divulges private information with intent to hurt others.
• noun slang Tattletale; squealer.
• noun Something that serves to reveal something else.
• noun music A movable piece of ivory, lead, or other material, connected to the bellows of an organ, whose position indicates when the wind is exhausted.
• noun nautical A length of yarn or ribbon attached to a sail or shroud etc to indicate the direction of the flow of the air relative to the boat.
• noun nautical A mechanical attachment to the steering wheel, which, in the absence of a tiller, shows the position of the helm.
• noun nautical 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.
• noun engineering A machine or contrivance for indicating or recording something, particularly for keeping a check upon employees (factory hands, watchmen, drivers, etc.) by revealing to their employers what they have done or omitted.
• noun A bird, the tattler.
• adjective revealing something not intended to be known

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective disclosing unintentionally
• noun someone who gossips indiscreetly