Origin and history of wag

early 13c., waggen (intransitive), "waver, vacillate, lack steadfastness," probably from a Scandinavian source (compare Old Norse vagga "a cradle," Danish vugge "rock a cradle," Old Swedish wagga "fluctuate, rock" a cradle), and in part from Old English wagian "move backwards and forwards;" all from Proto-Germanic *wag- (source also of Old High German weggen, Gothic wagjan "to wag"), which is probably from PIE root *wegh- "to go, move, transport in a vehicle."

From late 14c. as "be in motion back and forth or up and down." The transitive meaning "cause to move alternately back and forth or up and down" is from c. 1300; specifically in reference to dogs and their tails by mid-15c.: "and whanne they [hounds] see the hure maystre they wol make him cheere and wagge hur tayles upon him." [Edward, Duke of York, "The Master of Game," 1456]. Related: Wagged; wagging. Wag-at-the-wall (1825) was an old name for a hanging clock with pendulum and weights exposed.

"person fond of making jokes," 1550s, probably 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.); compare wagger "one who stirs up or agitates" (late 14c.). Related: Waggery "acts and words of a wag, mischievous merriment" (1590s).

A Wag is one that never in its life saw a beautiful object ; but sees, what it does see, in the most low, and most inconsiderable light it can be placed. There is a certain ability necessary to behold what is amiable and worthy of our approbation, which little minds want, and attempt to hide by a general disregard to every thing they behold above what they are able to relish. [Steele, "Tatler" no. 184, June 13, 1710]

"act of wagging, a shake," 1580s, from wag (v.).