Definitions

1. adjective

giving a false appearance of friendliness or innocence, but actually threatening or dangerous

“The sinister smile on his face sent shivers down my spine.”

2. noun

a person or thing that inspires fear or apprehension, especially because of their malevolent or evil nature

“The sinister figure lurking in the shadows seemed to be watching us.”

3. adverb

in a way that inspires fear or apprehension

“He walked through the abandoned house with a sinister air about him.”