c.1225, 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" (see
weigh).
Wagtail is attested from 1510 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.