The name "Hundred Poisons Unapproach Body" is not mentioned anywhere in the Novel.
This unique body grants total immunity to all poisons. Even poisons created by a Divine Body would have no effect on the person who has developed the Hundred Poisons Unapproach Body.
The immunity of this body is comparable, if not greater, than immunity of Poison Divine Body. This is apparent since the Poison Mist Formation set up by Poison Demon, is also unable to harm Chu Feng. Du Xiangyu is not even certain that Du Wanwu whose Divine body is of Poison Type will be able to pass the spirit formation safely and must blackmail Chu Feng to use a secret technique that is passed down in Du Clan to help Du Wanwu cross the Poison Mist Formation.
Chu Feng obtained this body as a result of surviving a dreadful experiment of Snow-haired Immortal using Poisonous Soul Devouring Bugs. Snow-haired Immortal performed the experiment on Chu Feng as a sort of punishment for losing a scroll she had Chu Feng safe-keep to Compass Immortal.
Snow-haired Immortal knows that after being bitten by the Poisonous Soul Devouring Bugs, one will be able to gain immunity from poisons. However, she is still not certain whether after one gained the immunity, one would able to withstand the poisons from the Extremely Poisonous Sea of Clouds, aka the Poison Mist Formation.
The experiment is done by having Chu Feng enter a powerful spirit formation and endure assaults from unknown numbers of Poisonous Soul Devouring Bugs. Snow-haired Immortal forbid Chu Feng to attack, defend, and oppose the bugs and only allow him to use whatever techniques he possessed to restore his internal organs, bones and meridians so as to prevent the bugs from reaching his Dantian.
The requirement to obtain this body is immensely high. One must have ability to quickly restore one's body unceasingly to keep up with the continuous, very fast, and simultaneous attacks from the armies of bugs. Even Chu Feng with the help of Vermilion Bird Revival Technique is having difficulties to keep up with the assaults of the bugs. Chu Feng has to fully concentrate on controlling his world spirit techniques and the Vermilion Bird Revival Technique to restore the portions of his body that had been devoured by the bugs. It has to be noted that the Vermilion Bird Revival Technique is a technique that is capable of instantly repair broken limbs. Up to date, Chu Feng is the only successful experimental subject.