This is a sly reference to and synonym for the penis or cock. For males adding the penis to the 10 fingers and 10 toes makes 21 appendages. Use this if you want to sound intelligent or don't want stupid or younger people to know what you are talking about.
I'm in the process of writing an epic poem about the love between me and my twenty-first appendage.
ayon kay Kyle Bravlavski AKA Jewboy ika-23 ng Disyembre, 2010