From Old English (circa 1550)
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.
Part or all of this entry has been imported from the 1913 edition of Webster’s Dictionary, which is now free of copyright and hence in the public domain. The imported definitions may be significantly out of date, and any more recent senses may be completely missing.
Related terms 
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- revealing something not intended to be known
- His 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.