wag, n.³ — Green’s Dictionary of Slang

wag n.³

[their wagging head, whether through stupidity or the desire to affirm whatever has been said]

1. a general term of contempt.

1890		‘’Arry on [...] the Glorious Twelfth’ in Punch 30 Aug. 97/2: ‘This swagger about killing birds is mere cant,’ sez this wobbling old wag.
1914		Examiner (Launceston, Tas.) 16 Nov. 7/5: Children who ought to be well educated are playing the part of the wag and the dunce.
1992		D. Burke Street Talk 2 71: What a wag!
1999		Observer Rev. 1 Aug. 10: Some wag shouts ‘What’s the singer called then?’.

2. anyone without firm opinions, a ‘yes-man’.

1997