It’s my fault, my mistake. The term was taken over from Latin as far back as the 1200s and continues to be used in this way. Ian Rankin had it in Strip Jack (1992), “‘You haven’t had a proper lock fitted yet.’ ‘Mea culpa, Inspector. Fear not, one’s on its way.’” A newer slangy version of this ancient Latin expression is my bad, only a few decades old but ubiquitous enough to be considered a cliché. Novelist John Lescroart used it in The Hunt Club (2009): “‘Yeah, you’re right, I’m sorry. My bad.’ Juhle hung his head.”