(See Absalom and Achitophel.)
Achitophel was David's traitorous counsellor, who deserted to Absalom; but his advice being disregarded, he hanged himself (2 Samuel xv.). The Achitophel of Dryden's satire was the Earl of Shaftesbury:
Of these (the rebels) the false Achitophel was first; A name to all succeeding ages curst; For close designs and crooked counsels fit; Sagacious, bold, and turbulent of wit; Restless, unfixed in principles and place; In power unpleased, impatient in disgrace.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894