Brewer's: Wag Beards
(To). “'Tis merry in hall when beards wag all” —i.e. when feasting goes on.
Then was the minstrel's harp with rapture heard; The song of ancient days gave huge delight; With pleasure too did wag the minstrel's beard, For Plenty courted him to drink and bite.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894