A Humping Party is when you and a whole bunch of friends (male or female) just start humping each other and other friends, random people, etc. While doing it you've gotta yell out "Humping Paaaaaaaaaaaaaaaarty" , to make it official.
Guy 1: Yo, Lisa is looking sexy today
Guy 2: Then lets throw a Humping Party
*Guys 1 and 2 walk over to Lisa and hump her in a friendly way*
Guys 1 and 2: HUMPING PAAAAAAAAAAAAAARTY.