To force someone to partake in an activity or to do something that is "voluntary" against their wishes. Normally this is because you, or no-one else in a group of people can be bothered to do the task in question.
"I forceunteer James to be the Guinea-Pig, He's the weakest of us all!"
"Bleh, I can't be bothered to walk the dog... Son, I forceunteer you to do it"