Something that's telltale accidentally relays important information. A wise parent can tell who ate the last of the birthday cake by the telltale chocolate smudges on the culprit's face.

The telltale dark circles under your teacher's eyes betray how little sleep he got last night, and a telltale bulge in a shopper's pocket is sometimes enough for a store owner to suspect she's a shoplifter. If it's telltale, its very presence reveals something. A famous occurrence of telltale, although it's spelled with a hyphen, is Edgar Allen Poe's story "The Tell-Tale Heart," about a murderer who imagines he's betrayed by the telltale beating of his victim's heart.

Definitions of telltale
  1. adjective
    disclosing unintentionally
    “a telltale panel of lights”
    “a telltale patch of oil on the water marked where the boat went down”
    synonyms: revealing, telling
    informative, informatory
    providing or conveying information
  2. noun
    someone who gossips indiscreetly
    synonyms: blabbermouth, talebearer, taleteller, tattler, tattletale
    see moresee less
    type of:
    gossip, gossiper, gossipmonger, newsmonger, rumormonger, rumourmonger
    a person given to gossiping and divulging personal information about others
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