where somebody cashes in on a joke you have made it is refered to as cashing in as the user is taking credit for a joke you have made and reaping the rewards from it. It is an unjust act of joke stealing
thomas pokes emily with a bannana and his group of friends find it hysterical.
phillip then repeats this act for the same reaction
thomas exclaims, "PHILLIP YOU'RE JUST CASHING IN!"