Well, there isn't a classical Latin word as they didn't have coffee or caffeine. That said, the Vatican puts out an updated Latin dictionary every year with words to cover modern things. I just don't know how to actually get a copy of it. (I know it exists because one of my favorite people at my high school had done things like label her fax machine with the official approved Latin.)

Stimulant is Stimulus, -i, m 1st delcension. I'd use that. (Okay, also irritamentum, irritamen, but I don't thik that has the connotation you're looking for.)

Coffeum is actually quite possibly correct. The 'new latin' words are generally not all that imaginative. No matter how amusing it is to say 'da mihi cocam-colam'.