Maybe I'm out of touch with the lingo, but I don't remember being told by the sweating, hyperactive guest lecturer in the gymnasium in 7th grade that alcohol, nicotine, and caffeine were hard drugs.
I do remember he told us a story about some girl snorting so much cocaine that she burnt a hole through to her brain, and her brain was leaking, and she had to have surgery transplanting skin from her butt up into her nose, and that now she walks around all day smelling her butt.
The diagram in the Wikipedia: Hard and soft drugs article asserts that alcohol and nicotine are hard drugs, and caffeine is a maybe. I suspect that this idea came from potheads.