E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
To pander to ones vices is to act as an agent to them, and such an agent is termed a pander, from Pan-drus, who procures for Troilus the love and graces of Cressida. In Much Ado about Nothing it is said that Troilus was the first employer of pandars (v. 2). (Shakespeare: Troilus and Cressida; Chaucer: Troilus and Cresseide.)
Let all pitiful goers-between be called to the worlds end after my name, call them all Pandars. Let all constant men be Trolluses, all false women be Cressids, and all brokers-between, Pandars. Say, Amen.Troilus and Cressida, iii. 2.