Webster's Revised Unabridged Dictionary · 1913

Wag

Wag , transitive verb

[Old English waggen; probably of Scand. origin; compare Swedish vagga to rock a cradle, vagga cradle, Icelandic vagga, Danish vugge; akin to Anglo-Saxon wagian to move, wag, wegan to bear, carry, German & Dutch bewegen to move, and English weigh. r136. See Weigh.]

To move one way and the other with quick turns; to shake to and fro; to move vibratingly; to cause to vibrate, as a part of the body; as, to wag the head.
No discerner durst wag his tongue in censure. — Shakespeare
Every one that passeth thereby shall be astonished, and wag his head. — Jer. xviii. 16

Wag expresses specifically the motion of the head and body used in buffoonery, mirth, derision, sport, and mockery.

Wag , intransitive verb

1.
To move one way and the other; to be shaken to and fro; to vibrate.
The resty sieve wagged ne'er the more. — Dryden
2.
To be in action or motion; to move; to get along; to progress; to stir. [Colloquial]
“Thus we may see,” quoth he, “how the world wags.” — Shakespeare
3.
To go; to depart; to pack oft. [Rare]
I will provoke him to 't, or let him wag. — Shakespeare

Wag , noun

[From Wag, v.]

1.
The act of wagging; a shake; as, a wag of the head. [Colloquial]
2.
A man full of sport and humor; a ludicrous fellow; a humorist; a wit; a joker.
We wink at wags when they offend. — Dryden
A counselor never pleaded without a piece of pack thread in his hand, which he used to twist about a finger all the while he was speaking; the wags used to call it the thread of his discourse. — Addison