The word sinister has two possible meanings. In its most common usage, it refers to something that is evil or threatening in nature. This can be applied to people who have malicious intentions or actions, as well as situations or objects that are dangerous or menacing.
In a more specific sense, the word sinister also means "left-handed" - this comes from an old belief that left-handers were associated with evil and wrongdoing because they used their left hand for activities such as writing with ink (which was considered unlucky), or warding off evil spirits.
In summary, the word sinister can be used to describe something that is threatening or dangerous, or someone who is left-handed - although its primary meaning is negative and refers to something bad or ominous.