Green’s Dictionary of Slang

mooner n.

[presumably f. the effect of the moon on the mind, or due to the effects of a full moon, during which time the crime rate supposedly increases]

(US Und.) a pathological lawbreaker.

[US]N.Y. Times Mag. 16 Mar. 88: Mooner. — One of a number of people who keep policemen busy during full moons (when, according to police lore, categories of crime show an increase) [HDAS].
[UK]J. Morton Lowspeak 100: Mooner – a psychopathic criminal, one who commits crimes such as rape.