telltale (n.)

Old English tellan "to reckon, calculate, number, compute; consider, think, esteem, account" (past tense tealde, past participle teald), from Proto-Germanic *taljan "to mention in order" (source also of Old Saxon tellian "tell," Old Norse telja "to count, number; to tell, say," Old Frisian tella "to count; to tell," Middle Dutch and Dutch tellen, Old Saxon talon "to count, reckon," Danish tale "to speak," Old High German zalon, German zählen "to count, reckon"), from PIE root *del- (2) "to count, reckon" (see tale).

Meaning "to narrate, announce, relate" in English is from c. 1000; that of "to make known by speech or writing, announce" is from early 12c. Sense of "to reveal or disclose" is from c. 1400; that of "to act as an informer, to 'peach' " is recorded from 1901. Meaning "to order (someone to do something)" is from 1590s. To tell (someone) off "reprimand" is from 1919.

Original sense in teller and phrase tell time. For sense evolution, compare French conter "to count," raconter "to recount;" Italian contare, Spanish contar "to count, recount, narrate;" German zählen "to count," erzählen "to recount, narrate." Klein also compares Hebrew saphar "he counted," sipper "he told."

Old English talu "series, calculation," also "story, tale, statement, deposition, narrative, fable, accusation, action of telling," from Proto-Germanic *talō (source also of Dutch taal "speech, language," Danish tale "speech, talk, discourse," German Erzählung "story," Gothic talzjan "to teach"), from PIE root *del- (2) "to recount, count." The secondary Modern English sense of "number, numerical reckoning" (c. 1200) probably was the primary one in Germanic; see tell (v.), teller and Old Frisian tale, Middle Dutch tal, Old Saxon tala, Danish tal, Old High German zala, German Zahl "number."

The ground sense of the Modern English word in its main meaning, then, might have been "an account of things in their due order." Related to talk (v.) and tell (v.). Meaning "things divulged that were given secretly, gossip" is from mid-14c.; first record of talebearer "tattletale" is late 15c.