telltale

1. One who informs on another; a talebearer.
2. Something that indicates or reveals information; a sign.
3. Any of various devices that indicate or register information, especially:
   1. A time clock.
   2. 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.
   3. A row of strips hung above a railroad track to warn a passing train of low clearance ahead.
4. 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.

noun

A person habitually engaged in idle talk about others: blab, gossip, gossiper, gossipmonger, newsmonger, rumormonger, scandalmonger, tabby, talebearer, taleteller, tattle, tattler, tattletale, whisperer. Slang yenta. See words.

ˈtelܖtāl n. (on a sailboat) a piece of string or fabric that shows the direction and force of the wind.

See the Introduction, Abbreviations and Pronunciation for further details.

Any device designed to indicate movement of formwork.

The noun has one meaning:

Meaning #1: someone who gossips indiscreetly
  Synonyms: tattletale, tattler, taleteller, talebearer, blabbermouth

The adjective telltale has one meaning:

Meaning #1: disclosing unintentionally
  Synonyms: revealing, telling