Technically a GiftGas (Poison Gas) used to execute concentration camp bound Jews, but in this sense a term coined by BubTheZombie on Microsoft's Halo 2 to refer to Plasma Grenades which were thrown onto Joobs in the gametype Showers.
BubTheZombie:"I GOT 4 CANS OF ZYKLON B, WHICH ONE OF YOU JOOBS WANTS SOME?!"
The brainchild of one EpicDarkness13 and one BubTheZombie, formed during the playing of a gametype on Xbox's Halo 2, called 'Showers' in which the Halo Secret Police, (In most instances Bub and Epic) command the "Joobs", or Jewish noobs, into room 1 of Foundation, where they are showered with Zyklon B, AKA Plasma Grenades.
EpicDarkness13:"Get to room 1 you fuckin' joob!"
BubTheZombie:"Look at the wall, Jewcake!"
*A Zyklon B canister has been showered upon the unlucky joob by now.*