shackin it up 

to have sex or get really comfortable with another person... usually of the opposite sex...

Koby wasn't just shackin it up with monika but with every other girl out there except for me cuz i dumped him!!!!

shackin it up by Koby's ex... same one as above June 26, 2003

shackin it up 

to get it goin on with another person... having sex...getting close and cuddly with each other

Monika and Koby was "shackin it up" with each other in the bed

shackin it up by Koby's ex June 26, 2003