Down Payment
When you really need to poop, but you're in the middle of doing something. Instead, you rush to the toilet to make a down payment, which means taking part of your poop and quickly returning to the activity that is occupying you. Usually, making a down payment implies that you will return to the toilet and make the rest of your "payment" later, but it doesn't have to.
Example:
Keith: Ben, I've got to take a dump.
Ben: But we're in the middle of a game of pool!
Keith: Don't worry, I'll make a down payment. Won't even take a minute.
Ben: *sigh* okay.
Keith: Ben, I've got to take a dump.
Ben: But we're in the middle of a game of pool!
Keith: Don't worry, I'll make a down payment. Won't even take a minute.
Ben: *sigh* okay.