According to the legend on Andy. Ninjahoppin is the act of spying on people in your neighborhood at night while wearing black clothes. To catch them masturbating of having sex.
by guywargamer May 30, 2005
Ninja Hoppin is when you go from house to house looking in people's windows like peeping toms, but when you find a couple having sex, be it strait or gay, you sneek up on them and join them, it ends up as a threeway gangbang.
by Sodapop April 03, 2006