'Left', or 'on the left side', in Heraldry; comes straight from the Latin. Note that this is from the point of view of the bearer; to someone looking at a shield as depicted, if something is sinister it is on the right-hand side. The fact that sinister has come to refer to something slyly threatening is one of the many remnants of anti-left-handed prejudice fossilised in our language, all suggesting that there is something just 'not right' about left.
The counterpart of sinister is dexter on the right. These two give their names to Sinister Dexter of the Principia Discordia and the main characters in Sinister Dexter from the comic 2000AD (Finnigan Sinister & Ramone Dexter).
'Sinister' is also the title of a long-running Belle and Sebastian mailing list; it is named after their first album, If You're feeling Sinister, which is often called Sinister for short. The web site for it is at http://www.missprint.org/sinister/ - bear in mind that it tends to be quite high-traffic, or anyway it was when I joined up for a while some time around 1999.