David E. Kaplan

“A complete back tattoo, stretching from the collar of the neck down to the tailbone can take one hundred hours. Such extensive tattooing, then, became a test of strength, and the gamblers eagerly adopted the practice to show the world their courage, toughness, and masculinity. It showed, at the same time, another, more humble purpose - as a self-inflicted wound that would permanently distinguish the outcasts from the rest of the world. The tattooing marks the yakuza as misfits, forever unable or unwilling to adapt themselves to Japanese society.”
― David E. Kaplan, Yakuza: Japan's Criminal Underworld