Used to describe pointless relationships that start when it is expected to last for a short period of time. It assumes that a couple has only a physical relationship and proceeding it, will breakup hence " hump then dump". If said quickly enough the term sounds like "humptydumpty" (that egg that sat on the wall but shattered because he fell...)
"I can't believe Larlar and that foreign exchange student are dating?!"
"Oh, it's just a hump'dendump relationship. He's going back to Turkey in a month"
"... he's so creepy.."
"Mom, how come I don't know my father?"
"He was executed a month after conceiving you."
"You dated a deathrow criminal?"
"It was a hump'dendump relationship"