Telltale

From Old English (circa 1550)

Full definition of telltale

(plural telltales)

One who divulges private information with intent to hurt others.
(chiefly US, slang) Tattletale; squealer.
Something that serves to reveal something else.The telltale was the lipstick on his shirt collar.
(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.
(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.
(nautical) A mechanical attachment to the steering wheel, which, in the absence of a tiller, shows the position of the helm.
(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.
(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.
A bird, the tattler.

(one who divulges private information) blabbermouth, gossip, rumormonger, talebearer

revealing something not intended to be knownHis eye was blinking, a telltale signal that he was lying.He blushed when he approached, a telltale sign that he was happy to see him.