The activity that the members of your social group will be participating in on a particular occasion. It may be stated, when asked, 'what's the motive?', the location where the motive will be taken place. Multiple activities are often stated when asked' 'what's the motive?'.
Posterior meaning buttocks, the phrase is derived from the phrase "ulterior motives". The phrase refers to a male being phony twords the opposite sex with the intentions of sexual Intercourse.
Seeing a male explaining any of the following to a female could be seen as having Posterior Motives: "I'm different than the other guys", "I love poetry!", "I think you look great with no makeup" "I think you are so unique" ect ect.