Having a strange, mysterious, or bewitching quality; alluring or enchanting. Can also sometimes imply suspiciousness depending on context.
夜道であの建物から漏れる光がとても妖しかった。
The light leaking from that building on the night street looked very eerie.
彼女は妖しい微笑みを浮かべて部屋に入ってきた。
She entered the room with an enchanting/suggestive smile.
その話にはどこか妖しい点があって、私は信じられなかった。
There was something dubious about that story, so I couldn't believe it.