A child who is conceived through a good fucking as opposed to an act of love. Often used after the word bastard to strengthen it - bastard fucklet - , thereby implying the child was not only conceived out of wedlock, but also from a loveless screwing - a good fucking.
Could just be a simple insult that a joe six-pack might shout at his child:
Go get me another beer, you little fucklet.
If that same child scratched his father's Pontiac Firebird.
Fucker: You two fucklets play together while I fuck mommy some more.
......later that day:
Fuckress: Oh yeah, baby, lets make some more fucklets.
......later that year:
Fucker: You three fucklets play together...etc.