"Rump" normally means the hind end of an animal or backside; its use meaning "remnant" was first recorded in the above context in English. Since 1649, the term "rump parliament" has been used to refer to any parliament left over from the actual legitimate parliament.
In September 1648, at the end of the Second English Civil War, the Long Parliament was concerned with the increasing radicalism in the New Model Army. The Long Parliament began negotiations with King Charles I. The members wanted to restore the king to power, but wanted to limit the authority he had. Charles I conceded militia power, among other things, but he later admitted that “it was only so he could escape”. In November the negotiations began to fail, and the New Model Army seized power. Charles I was then taken into the Army’s custody, to await trial for treason.
The New Model Army wanted to prevent Parliament from agreeing on the Treaty of Newport to reinstate King Charles I. While Presbyterian and moderate elements within Parliament were inclined to continue negotiations, the Army was impatient with Charles. Thomas Fairfax, by issuing a command to Commissary General Ireton, organized a military coup in 1648. Ireton intended to dissolve the Long Parliament but was persuaded to purge it instead. He then ordered Colonel Thomas Pride to prevent the signing of the Treaty of Newport.
Between 6 and 12 December, Pride—supported by two regiments—prevented 231 known supporters of the treaty from entering the House, imprisoning 45 for a few days. The remaining free members then became the Rump Parliament.
When it became apparent to the leaders of the New Model Army that Parliament—then controlled by the Presbyterian faction—was ready to come to an agreement with the King that would restore him to the throne (though without effective power) and negate the power of the Army, they resolved to shatter the power of both King and Parliament. Pride's Purge brought Parliament to heel under the direct control of the Army; the remaining Commons (the Rump) then on 13 December 1648, broke off negotiations with the King. Two days later, the Council of Officers of the New Model Army voted that the King be moved from the Isle of Wight, where he was prisoner, to Windsor, "...in order to the bringing of him speedily to justice". The King was brought from Windsor to London in the middle of December.