A child who is conceived through an act of rape. Clearly a step and a half worse than being a common little bastard or fucklet.
Could just be a simple insult that a joe six-pack might shout at his child:
Give me the TV remote you little rapelet.
or, it could be used as defined
Jimmy's mama was raped down by the train tracks by a drunken bum, and the bum knocked her up. Therefore, Jimmy is a rapelet.