in Dryden's satire of Absalom and Achitophel, is meant for Richard Cromwell. His father, Oliver, is called Saul. At the death of Saul, Ishbosheth was acknowledged king by a party, and reigned two years, when he was assassinated. (Part i. 57, 58.)
They who, when Saul was dead, without a blow, Made foolish Ishbosheth the crown forego.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Ishbosheth from Fact Monster: