A feeble-minded old man, the foil of the clown, whom he aids and abets in all his knavery. The word is derived from the dress he used to wear, a loose suit down to the heels.
“That Licentio that comes a-wooing is my man Tramo bearing my port, that we might beguile the old pantaloon.” —Shakespeare: Taming of the Shrew, iii. 1.
Pantaloon. Lord Byron says the Venetians were called the Planters of the Lion- i.e. the Lion of St. Mark, the standard of the republic; and further tells us that the character of “pantaloon,” being Venetian, was called Piantaleone (Planter of the Lion). (Childe Harold, bk. iv. stanza 14, note 9.)
Playing Pantaloon. Playing second fiddle, being the cat's-paw of another; servilely imitating.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894