c. 1200, "recitation of the 51st Psalm" (in Vulgate, the 50th), one of the "Penitential Psalms," so called from the phrase Miserere mei Deus "Have mercy upon me, O God," the opening line of it in the Vulgate, from Latin miserere "feel pity, have compassion, commiserate," second person singular imperative of misereri "to have mercy," from miser "wretched, pitiable" (see miser).
From 15c.-17c. it was used as an informal measure of time, "the time it takes to recite the Miserere." The musical settings of the psalm are noted for their striking effectiveness. The Latin verb also is in miserere mei "kind of severe colic ('iliac passion') accompanied by excruciating cramps and vomiting of excrement" (1610s); literally "have mercy on me."