a statement used when a situation becomes either undeniably irreversible or utterly impossible to explain due to temporary unfavorable circumstances. Often preceded by the "it's not what it looks like" statement. In other words: you're busted and you can't come up with a good excuse without getting beat up or yelled at.
A girl comes to a guy's house for a visit. They enjoy a good conversation. They turn on the tv. The air conditioner doesn't work so they take off a few items of clothing. They have a friendly fight over the remote and start wrestling on the ground. The guy's overzealous girlfriend walks in to see them holding eachother, nearly half naked on the carpet. The guy is shocked and says "Wait, it's not what it looks like...I can explain."