Nokogiri is a wooden sword, light brown with a dark brown lacquer in a wave-like pattern somewhat resembling a saw.
Nokogiri is made of wood and unfit for a battlefield (though, as noted, can still be used to injure or kill). Its strength lie elsewhere: it is the only deviant blade that is free of "poison" and is said to have the power to cleanse "poison" left by other swords. In addition, wielding the blade grants the user "Royal paradise", a state of inner peace and calm, thus removing the need to fight.
Zanki Kiguchi became aware of hypocrisy calling such a weapon peaceful when it could be used to grievously injure or kill and would be willing to surrender the sword if not for the honour of the dojo. The shogun's retainer referred to achieving inner calm, but approached Shichika with killing intent, promising swift and merciful death (while being aware that a blunt wooden tool would never grant such thing), thus making him remark that his words are empty and defeating the retainer effortlessly.
Nokogiri means saw, referring to the sword's lacquer pattern. Its title, Royal, refers to its status as "governing" over other swords, it is also the most noble blade.