A frenzied practitioner of consensual BDSM involving repetitive blows ("spanks") rained down upon the voluntarily uplifted bottom. By extension, someone who can't stop talking about this hobby.
Suzeemoon much enjoyed being disciplined by her Cariad using well-judged slaps upon her gluteus maxima - in fact, she was an incorrigible Spank Monkey.
Someone who's life has been consumed by internet porn, and they spend all their time beating off in front of the computer.
Tommy turned into a spank monkey when his family got high speed internet service.