Mea culpa is a Latin phrase that translates into English as "my mistake" or "my fault". To emphasize the message, the adjective "maxima" may be inserted, resulting in "mea maxima culpa," which would translate as "my most fault."
The origin of the expression is from a prayer of confession of sinfulness used in the Mass of the Roman Catholic Church known as Confiteor (Latin for "I confess"), of which the first evidence date from shortly before 1100. The phrase "mea culpa" appears in the prayer from the 16th century. The Ordinary Form of the Roman Rite text of the prayer is (with the new approved 2010 ICEL English translation, soon to be put into use) :
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,