Mea culpa
The phrase comes from a prayer of confession of sinfulness, known as the "Confiteor", used in the Roman Rite at the beginning of Mass or when receiving the sacrament of Penance. Grammatically, "" is in the ablative case, with an instrumental meaning. Religious use. At the sight of the crucifixion of Jesus in the Gospels, "the whole crowd who came together to that sight, seeing what had been done, beat their breasts and returned"." In the present form of the "Confiteor" as used in the celebration of Mass, "mea culpa" is said three times, the third time with the  addition of the adjective "maxima" ("very great", usually translated as "most grievous"), and is accompanied by the gesture of beating the breast. According to Adrian Fortescue, the inclusion in the "Confiteor" of the phrase "mea culpa" can be traced back only to the 16th century. However, the Latin phrase "mea culpa" was used, even in an English context, earlier than that. Geoffrey Chaucer's 14th-century "Troilus and Criseyde" uses it in a way that shows it was already a traditional religious phrase: "Now, "mea culpa", lord! I me repente." Although the "Confiteor" was not then part of the Mass , it was used in administering the sacrament of Penance. In some forms it already included the phrase "mea culpa". Thus the 9th-century "Paenitentiale Vallicellanum II" had a thrice-repeated "mea culpa" (without "maxima") in its elaborate form of the "Confiteor". In about 1220, the rite of public penance in Siena for those who had committed murder required the penitent to throw himself on the ground three times, saying: "Mea culpa; peccavi; Domine miserere mei" ("Through my fault. I have sinned. Lord, have mercy on me").