|Hassan-i Sabbah, the Grandmaster of the Order of Assassins|
[French, or medieval Latin assassinus from Arab hašîšî hasish-eater]
A person who undertakes to assassinate somebody;
Any of a band of Ismaili Muslim fanatics in the time of the Crusades who were allegedly sent on murder errands by the Old Man of the Mountains or by later leaders.
I first came across an explanation for assassin's etymology in The Da Vinci Code. Knowing the risk of taking any "fact" in that book at face value, I looked it up in several sources and, lo and behold, there does seem to be some shred of historicity to it. Well done you, Dan Brown.
However, things do get a little hazy, as it seems in doubt that the early assassins actually ate hasish before setting out on a mission to kill someone. Some sources claim this was actually something of a slur campaign, and the term in Arabic for hasish-eater actually meant rabble or social outcasts. Although it was in use earlier, it came to be applied to the Ismailis by those that didn't hold them in high regard, and perhaps was equivalent to branding some fringe group with extreme ideas as a bunch of potheads, whether they're on the toke or not.
What does seem to be without doubt, however, was that there was an Order of Assassins, dating from around the First Crusade in the 11th century and lasting up until their destruction by the dastardly Mongols in 1256. This Order carried out, surprise surprise, assassinations against various targets, both Muslim and Christian. Targets were usually public figures, or those with influence, and the hits were usually carried out in public to maximise intimidation, which is pretty savvy for a bunch of stoners. They also tended to avoid harming civilians, and preferred precise, clean kills rather than widespread bloodshed, which is jolly good of them.
Lastly, although they could be roughly considered synonymous, there does seem to be a popular difference between an assassin and a hitman. Basically, an assassin in a person that kills primarily for some ideological purpose, whereas a hitman is primarily professional - they're in it for the money.
If you'd like to read more about the Order of Assassins, you can have a look at the Wikipedia page. Please note, though, that they're no longer hiring, so those interested in a career as an assassin will have to look elsewhere.