A tattletale is a name given to somebody when they report others wrongdoings without discretion. Somebody who gossips does it for attention, tattling is the same thing.
I can sadly say I’ve spent hours searching for this answer, and so far have not came up with any relevant information. I think it’s just one of those words we just have to accept that it doesn’t really come from anywhere; it was just there, although I would love to be proved wrong on this occasion. If anyone does happen to find out where it originates from, please don’t be afraid to tell me, I award cookies to anyone who helps when I ask.
And Abby, stop being such a tattletale!
Update: "The word tattletale is a compound word that owes its roots to several languages. To tattle is to tell and is derived from the Flemish verb tatelen which means to stutter, and the Frisian term tateren which translates to “to tell tales or secrets.” Talo is a Germanic term related to the words talk and tell. When put together, as they were first in the 15th century, a tattletale is someone who betrays secrets or “tells on” others." - Thanks to James for solveing this.