51st Psalm (one of the Penitential Psalms), 13c., from Miserere mei Deus "Have mercy upon me, O God," opening line, from Latin miserere "feel pity, have compassion, commiserate," imperative of misereri "to have mercy," from miser (see miser). From 15c.-17c. used as an informal measure of time, "the time it takes to recite the Miserere." Also in miserere mei "kind of severe colic ('iliac passion') accompanied by excruciating cramps and vomiting of excrement" (1610s), literally "have mercy on me."