according to ernest it stems from /sbin. though one of the early instances was named karabin based on the karabine gun and thus shortened to kbin (or rather expanded from kbin).
Ernest has actually said that the name "magazine" isn't gun related and that it's a reference "to the virtual edition of a late 90s gaming newspaper" (see this thread and my other comment for the full story)
Someone in that thread also explains that in Polish the names are different:
Polish word "magazyn" means storage or newspaper, but "magazynek " is a gun mag.