Word Origin & History
early 13c., probably from a Scand. source (cf. O.N. vagga "a cradle," Dan. vugge "rock a cradle," O.Swed. wagga "fluctuate"), and in part from O.E. wagian "move backwards and forwards;" all from P.Gmc. *wagojanan (cf. O.H.G. weggen, Goth. wagjan "to wag"), probably from PIE base *wegh- "to move about"
). Wagtail is attested from c.1500 as a kind of small bird; 18c. as "a harlot," but seems to be implied much earlier:
"If therefore thou make not thy mistress a goldfinch, thou mayst chance to find her a wagtaile." [Lyly, "Midas," 1592]
Wag-at-the-wall (1825) was an old name for a hanging clock with pendulum and weights exposed.
"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