Telltale means giving away something that was meant to stay hidden. Telltale signs, a telltale mark, the telltale smell.
It is nearly always followed by signs, and it belongs to detection: a telltale sign is small evidence of something bigger. As a noun it is a child's word for somebody who reports on others.
1adjectivetelltaleGiving away something that was meant to stay hidden.
“The telltale smell of smoke on his coat showed he had been smoking again.”
telltale is a quality.
Tell plus tale, perhaps altered from the older taleteller.
Middle English
tale tellere
English, today
telltale
From tell + tale, perhaps dissimilated from earlier taleteller, from Middle English tale tellere literally “tale teller”.
Etymology from Wiktionary, CC BY-SA