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.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894