Mea Culpa
Mea Culpa is a Latin phrase that translates into English as "my fault", or "my own fault". In order to emphasize the message, the adjective "maxima" may be inserted, resulting in "mea maxima culpa", which would translate as "my [most] grievous fault".
The origin of the expression is in a part of the Catholic mass known as Confiteor (Latin for "I confess"), in which the individual recognizes his or her flaws before God. Despite it's appearance, the "mea culpa", as the Confiteor has come to be known popularly, is not a confession of sins, but rather an admission of one's flawed nature and the willingness to make amends for it.
The text in Latin is:
Confiteor Deo omnipotenti,
beatæ Mariæ semper Virgini,
beato Michæli Archangelo,
beato Ioanni Baptistæ,
sanctis Apostolis Petro et Paulo,
omnibus Sanctis, et vobis, fratres (et tibi pater),
quia peccavi
nimis cogitatione, verbo et opere:
mea culpa,
mea culpa,
mea maxima culpa.
Ideo precor beatam Mariam
semper Virginem,
beatum Michælem Archangelum,
beatum Ioannem Baptistam,
sanctos Apostolos Petrum et Paulum,
omnes Sanctos, et vos, fratres (et te, pater),
orare pro me ad Dominum Deum nostrum.
Amen.
This translates into English as:
I confess to Almighty God,
to blessed Mary ever Virgin,
to blessed Michael, the Archangel,
to blessed John the Baptist,
to the holy Apostles Peter and Paul,
to all the Saints and to you, Father,
that I have sinned exceedingly,
in thought, word and deed,
through my fault,
through my fault,
through my most grievous fault.
Therefore I beseech the blessed Mary,
ever Virgin,
blessed Michael the Archangel,
blessed John the Baptist,
the holy Apostles Peter and Paul,
all the Saints, and you, Father,
to pray to the Lord our God for me.
Amen.
As part of the prayer, it is customary for the faithful to strike gently at their own chests when pronouncing the "mea culpa" (or "through my fault", in the English version) section of the Confiteor. This is done to emphasize the admition of one's own sinful nature.
Popular meaning
In the popular vernacular, the expression "mea culpa" has acquired a more direct meaning, in which, by doing or performing a "mea culpa", someone admits to have made a mistake by one's own fault (meaning that it could have been avoided if that person had been more diligent). It may be used even in trivial situations: if a football player, for instance, admits that his team lost a match because he missed a penalty kick, this may be called a "mea culpa", meaning that he admited his mistake, which he could have avoided (at least in theory), and that resulted in a subsequent evil.