Some one who is being rude, unattractive or obnoxious.
"Wow, did you see that guy wearing pajamas at the mall?"
"Yeah, what a sinister doily."
When you eat a whole orange with its shell on, you refer to the orange as a sinister shell.
"Dude I was bored as hell and ate a sinister shell"
My man's girl was coming after me behind his back, that bitch is sinister.
If somebody really is good natured, they don't invite hostility and aren't involved in hostile events/incidents/arguments often (even ones they describe to others as hilarious situations). If someone is overly argumentative and does everything to excess, they are not entirely good if they are good at all.
The dork claimed to be a good college girl, but anybody with as much to run from as she had to run from must not have been entirely good, if there was any truth to her claim of being good at all (since it was a questionable claim where evil or sinister was just as likely to be her true nature).
Something that is far too demonic, use this word to emphasize such demonic nature
A band that is so amazing and can literally get any chick except for the United attractive ones.
Did you see that sinister concert the lead singer is so amazing.
Sinister is an amazing band that get all of the chicks except the non attractive ones.
Omg I love sinister and the lead singer is hot.