A highly toxic substance with origins in Australia. Immunity from the deadly properties of iocane can be built up over a period of time if it is introduced in minute portions.
As read/seen in "The Princess Bride": the name was most likely derived from a slightly altered anagram for cocaine, a commonly used drug during the time the book/movie were written (1973, 1987 respectively)
Buttercup: "And to think, all that time it was your cup that was poisoned!"
Man in black/ Westley : "They were both poisoned. I've spent the last few years of my life building up an immunity to iocane powder."