telltale

One who divulges private information with intent to hurt others.

Your husband is at hand; I heare his Trumpet, / We are not tell-tales, Madam; feare you not.

There were some ill-natured people —tell-tales—it seemed, in Tamai; and hence there was a deal of mystery about getting up the dance.

An indicator, such as a warning light, that serves to warn of a hazard or problem.

A railroad company, which has properly erected a “telltale,” or signal board, to give warning of the approach of a train to a bridge and which lights the place at night by an electric light, is not responsible for injury caused at night by the “telltale” to a brakeman who had been passing under it at regular intervals for five weeks, though when injured he was standing upon a new car higher than those before used, and though he ad received no warning from the company as to the danger of standing upon such a car while passing under the "telltale".

A float telltale to be provided which shall connect with indicator on first floor by a small chain run through 1-2 inch pipe and over pulleys well guarded against the weather.

Something that serves to reveal something else.

The telltale was the lipstick on his shirt collar.

It supplies many useful links and tell-tales.

A movable piece of ivory, lead, or other material, connected to the bellows of an organ, whose position indicates when the wind is exhausted.

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.

The sailmakers mistake can be an excellent way to lose a race as the telltale signals were disturbed and confusing.

If you turn toward the wind, the sail becomes undertrimmed, and the inside, or windward, telltale starts to rise and flutter as the flow is disturbed.

Revealing something, especially something not intended to be known.

I noticed the snow was dirty, a telltale sign of recent human presence.

A telltale blush crept into her cheeks as he approached.