A tall tale or a tall story was a story about a real person, the facts of which were questionable.
In 2268, Captain James T. Kirk offered to tell Ensign Garrovick several tall stories about his deceased father, Captain Garrovick. (TOS: "Obsession")
A biography on Davy Crockett that Julian Bashir gave Miles O'Brien in 2375 included several tall tales. Bashir related one in particular, in which "Crockett was supposed to have put up a target against a tree and arranged a series of tin frying pans nearby in a complicated pattern. The idea was that he was going to shoot a bullet at one of these pans which would then ricochet to another and then to another and then to another and then to another." Hearing this tale gave O'Brien a revelation as to why and how a murderer had killed several of Deep Space 9's crew with a TR-116 rifle - namely, via displaced targeting. (DS9: "Field of Fire")