one who tells on somebody when what they're telling doesn't at all affect them
Joe is such a tattle-tail. He saw me sneak a cookie and he told mom.
Specifically a person, usually a man, who blabs to their buddies about a woman/man they slept with, where the person they slept with specifically does not want anyone to know about the interaction or relationship. This could be for many reasons, such as keeping a work relationship a secret or cheating on a spouse. Not to be confused with tattletale, tail is specifically in reference to "getting some tail".
My ex was such a tattletail! He told his brother about us and now my parents know I'm not a virgin!