Definition of sinister

1: =singularly evil or productive of evil=

2: accompanied by or leading to disaster

3: presaging ill fortune or trouble

4a: of, relating to, or situated to the =left= or on the left side of something

especially : being or relating to the side of a heraldic shield at the left of the person bearing it

b: =of ill omen by reason of being on the left=

https://www.merriam-webster.com/dictionary/sinister