A so-called carpet knight was a person who had been awarded a title of knighthood by the king of England on a holiday occasion (or in time of peace), as opposed to knighthoods awarded for military service, or success in tournament games.
Notes and Queries explained in 1862:
The carpet knight is a term characteristically applied to those who obtained their honours "with unhacked rapier and on carpet consideration"... amidst the holiday gifts of their sovereign, rather than bravely acquired on the field of battle, or boasting a prescriptive claim by proving victorious at a tournament.
References
Knight of the Carpet Wikipedia(Text) CC BY-SA