wagtail
n.
c.1500, kind of small bird that has its tail in continuous motion (late 12c. as a surname), earlier wagstart (mid-15c.), from wag, v. + tail, n.. From 18c. as "a harlot," but this sense 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]