A parchment on which the original writing has been effaced, and something else has been written. (Greek, palin, again; psao, I rub or efface.) When parchment was not supplied in sufficient quantities, the monks and others used to wash or rub out the writing in a parchment and use it again. As they did not wash or rub it out entirely, many works have been recovered by modern ingenuity. Thus Cicero's De Republica has been restored; it was partially erased to make room for a commentary of St. Augustine on the Psalms. Of course St. Augustine's commentary was first copied, then erased from the parchment, and the original MS. of Cicero made its appearance.
“Central Asia is a palimpsest; everywhere actual barbarism overlays a by gone civilisation” —The Times.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894