to tell someone in authority about something bad that someone has done because you want to cause trouble for them (often + about ) She wasn't very popular at school - she was the sort of kid who was always telling tales about other kids.I had half a mind to tell my boss about him but I didn't want her to think I was telling tales.
Divulge secrets, as in Don't trust him; he's apt to tell tales. This expression was first recorded about 1350. A variant, tell tales out of school, first recorded in 1530, presumably alluded to schoolchildren gossiping but was soon broadened to revealing secret or private information. Both may be obsolescent.