Neither the OED and Etymonline has any answer to the etymology of the word. The latter does suggest it may have an origin in the Caribbean, but offers nothing better.
The first citation is from 1936 only. Here are the last two citations:
- 1975 High Times Dec. 137/1 — Like Marley, he’s a spliff-toking Rastafarian.
- 1977 Transatlantic Rev. lx. 192 — Coon rolled a massive splif and blew clouds of ganja gremlins through the hatch.
What’s the origin of the word spliff? Might there be precursor that makes sense only if you know the various creoles of the West Indies, or even just the Rastafarian culture?