a tall tale; a fish story; a big talk; hot air; one’s (often exaggerated/dubious) story/brag about his/her audacious or glorious experience in his/her youth
Originally, it refers to a heroic and brave story of someone famous, but nowadays it more often describes far-fetched stories told by middle-aged~old blokes at Izakaya, such as “You know what? Everyone fell in love with me back in high school” and “I often had a flight with yobs and beat ’em up”. Therefore, while not always, it often has a negative connotation.
Again, he started telling his tall tale about having dated five girls simultaneously back in high school.