So we just happened to start talking about this and I thought posting it was imperative, since it's a real useful word.
Zafacón is the word for trash can. See, I told you it'd be useful. Supposedly it comes from the English "safety can", which is, as far as I can tell, a name for trash cans in labs.
We theorized that, since there are so many pharmaceuticals on the island, and they probably had special "safety cans" for the chemicals, they changed the English name into Spanish. Maybe.