Mea culpa

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 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.

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.

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
cogitatióne, verbo,
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.

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."

...
Wikipedia