The story you may have heardTwo stories circulate. The hashish story, that the Assassins killed while high on hashish or were drugged with promises of paradise, comes from their enemies and from Marco Polo-era legend; no Ismaili source supports it, and the name was an insult. Amin Maalouf's claim that Hassan-i Sabbah called his followers asāsiyyūn 'faithful to the foundation' is repeated online (and by Wiktionary), but standard dictionaries derive the word from ḥašīš.