The Collaborative International Dictionary of English v.0.48:

Peccavi \Pec*ca"vi\ [L.]
   I have sinned; -- used colloquially to express confession or
   acknowledgment of an offense; -- used rarely, superseded by
   the approximate equivalent mea culpa. --Aubrey.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

mea culpa
    n 1: an acknowledgment of your error or guilt