An activity to which the person (usually male) proceeds to force a a 5 cent coin down their urethra (dick hole) then wanks and cums with the coin still lodged in there, the challenge is complete when the person has successfully climaxed and then also pulls the coin out.
John: I'm gonna do the 5 cent challenge tonight, wannado it with me?
Bob: no fucken way cunt, that's fucked
John: it's your loss it's actually quite pleasurable
Bob: whatever you say
Can be written as 五毛党, 五毛 and 5毛. It's an insult word for the Chinese Communist Party used by anti-Communist/pro-democracy people. The word is banned on a video hosting site, Youtube, along with Gongfei (CCP gangsters or CCP bandits).
Me : Typing 共匪 (Gongfei) and 5毛 (Wumao; 5-cent party) into Youtube comment section of a random video
*15 seconds later*
Me : My comments disappear. It seems they really support CCP liars enough to delete words like these.