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Mea culpa is a Latin phrase that means "through my fault" and is an acknowledgement of having done wrong.
Grammatically, meā culpā is in the ablative case, with an instrumental meaning.
The expression is used also as an admission of having made a mistake that should have been avoided, and may be accompanied by beating the breast as in its use in a religious context. If, for instance, a sports player admits that his team lost a game because he missed an opportunity to score, this acknowledgement may be called a mea culpa.
In the United States, "My bad" might be used in such circumstances.
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.
Confíteor Deo omnipoténti
et vobis, fratres,
quia peccávi nimis
ópere et omissióne:
mea culpa, mea culpa,
mea máxima culpa.
Ideo precor beátam Maríam semper vírginem,
omnes angelos et sanctos,
et vos, fratres,
oráre pro me ad Dóminum Deum nostrum.
I confess to almighty God
and to you, my brothers and sisters,
that I have greatly sinned,
in my thoughts and in my words,
in what I have done and in what I have failed to do,
through my fault, through my fault,
through my most grievous fault;
therefore I ask blessed Mary ever-Virgin,
all the Angels and Saints,
and you, my brothers and sisters,
to pray for me to the Lord our God.
Many different forms of the Confiteor have been in use over the centuries. That adopted for the Roman Rite in 1969 is as follows
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").
- "The Order of Mass". Universalis.com. Retrieved 24 February 2015.
- Fortescue, A. (1908). "History of the confiteor" in Catholic Encyclopedia. New York: Robert Appleton Company. Retrieved March 4, 2009
- Troilus and Criseyde, Book II, 525
- Wasserschleben, Friedrich Wilhelm (1851). Die Bussordnungen der abendländischen Kirche. Halle: Ch. Graeger. p. 555. Retrieved 24 February 2015.
- Schmitz, Herm. Jos. (1898). Die Bussbücher und das Kanonische Bussverfahren, vol. 2. Düsseldorf: L. Schwann. pp. 53–54.
- Sancta Missa – Prayers at the Foot of the Altar, Latin and English, sanctamissa.org