“Telling tales out of school” means to reveal confidential or sensitive information, or to gossip, or betray confidences.
The first published reference in the Oxford English Dictionary is from a 1546 book of proverbs and epigrams collected by the English writer John Heywood in Dialogue Containing the Number in Effect of all the Proverbs in the English Tongue: “To tell tales out of schoole, that is hir great lust.”
The phrase was originally used only for children, those who repeated the gossip that they’d heard at school to their family at home. That certainly still happens, and we would call them tattletales, but now it also applies to anyone who reveals confidences he has received. The saying was used by William Tyndale in The Practyse of Prelates: “So that what cometh once in may never out, for fear of telling tales out of school.” (From Dictionary of Cliches by James Rogers)
Telling tales out of school has always been a crime, because it breaks the bonds of fellowship and mutual support.
There was a snide little report in The Athenaeum, Aug 17, 1833: “School politics in Prussia. — The Prussian State Gazette contains an ordinance prohibiting all talking about politics in schools. We are unable to inform our readers whether riding-schools and schools for scandal be included in this prohibition. It, however, appears to us that it would have been much wiser to have prohibited politicians from talking out of school.” “Schools for scandal” refers to a Sheridan play; “riding school” was a contemporary term for a brothel.
The term “tattletale,” which first appeared in print in 1889, is actually drawn from “tell-tale,” an older (around 1548) term defined by the Oxford English Dictionary as “one who idly or maliciously discloses private or secret matters.”
The “tattle” comes from the verb “to tattle,” which originally, in the 1400s, meant to stammer or speak in baby-talk, but later came to mean “to gossip” and “to freely reveal secrets and private affairs.”
A much newer phrase, and one I’d never heard, is “drop a dime on,” meaning “to inform on or to betray” someone. This one first appeared in street slang in the mid-1960s, when it only cost a dime for a public telephone to make an anonymous call to the police.
Telling tales out of school, or being a tattletale is no doubt very bad. But I still like some juicy gossip now and then.