Ganglion cyst, aka "Bible bump", aka "Let's hit you with a large book!"
Given the name "Bible cyst" or "Bible bump" from the original "treatment" of hitting the deformity with a Bible or other large book, ganglion cysts are actually considered tumors. Over 60% of the non-cancerous tumors of the hand are ganglionic, but their etiology is still not completely understood.
The first description of these tumors was given by Hippocrates as "knots of tissue containing mucoid flesh", but the first theories of their origins weren't put forth until 1746, when one Dr. Eller wrote a descriptive paper on them. He believed that they came from synovial herniation, or rupture through the tendon sheath, and that they were true cysts.
Later theories recognized that the tumors were growths arising from ganglionic sheaths and not encysted fluids or mucoid tissues, but the old theory of "just smack it with a heavy object to burst it and make the body reabsorb it" still stuck around for a long time. Though bursting the tumor can cause temporary disappearance, this often causes more serious recurrence and additional growth. As these tissues are not cancerous, it's generally recommended to leave them alone when they're not interfering with normal function. Sometimes they can press against other tendons or nerves and necessitate aspiration or surgical removal, but the incidence of recurrence is over 40%. In about 75% of cases, the "cyst" will disappear or significantly decrease on its own.
So, yeah, don't let anyone convince you to let them smack you with a heavy object, just because you have one of these growths! It's a bad idea to let people smack you with heavy objects, just as a general rule...
[Source: Medscape Reference: Ganglions]
[Image: Iconograms. Prof. Bockenheimer, 1913.]