A sex offender with adult victims, i.e. a rapist. The term "NONCE" doesn't necessarily
mean child victims. It originated from Wakefield Prison in the UK and stood for "Not on Normal Courtyard Exercise." If a prisoner had committed horrific crimes such as child
murder, rape or pedophilia they would get a "NONCE" sign on the
door of their cell. This meant that the prison guards would not open their cell doors at the same
time as the other prisons because they were very likely to get beaten up by the other inmates. It doesn't necessarily mean pedophiles.
An adult nonce is essentially a sex offender who has victims that are adults. For example a serial rapist or woman murderer but the victims are adults.