Serious sexual activity. Hardcore and very fast moving with the testicles slapping up against the "taint" area of the female from the seriously fast sex. Usually occurs the most during doggy-style or plain 'ol hyperspeed missionary.
Wow, me and Mandy were having some serious sex, slappynutz were in full effect and I had sweat just fallin off me and my balls. We must have lost 10 pounds each from that 45 minute sexual hurricane.