Chroniclers including Marco Polo claimed that the followers of Sabbah committed their political murders while under the influence of drugs, hence the derogatory nickname.
However, this etymology may well have arisen after the name itself, as a creative attempt to explain its origins.
In any case, Hasan-i Sabbah strictly interpreted the Koran's injunction against intoxicants.
A more convincing explanation cites the Egyptian Arabic word hashasheen, meaning "noisy people" or "troublemakers."