My Bad: It is my fault, my mistake
It was my bad that the car door was left unlocked.
From phrases.org.uk: “My bad” is American in origin and was popularized by basketball players in the 1970s and 1980s. It circulated in urban street language until it gained widespread popularity. It picked up speed in the mid to late-1990s in the USA via the 1995 movie “Clueless”. This starred Alicia Silverstone and contains what seems to have been the first use of the phrase in the mainstream media. The 1994 ‘Green revision pages’ for the movie script has a scene with Alicia Silverstone’s character learning to drive:
“Cher swerves – to avoid killing a person on a bicycle. Cher: Whoops, my bad.”