Английский Этимологический словарь - wag (n.)

- "person fond of making jokes," 1553, perhaps a shortening of waghalter "gallows bird," person destined to swing in a noose or halter, applied humorously to mischievous children, from wag (v.) + halter. Or possibly directly from wag (v.). Wagtail is 1510 as a kind of small bird; 18c. as "a harlot."