Looking up...
Having a dark, threatening, or evil quality; sinister or ominous.
El personaje de la película tenía una sonrisa siniestra.
The character in the movie had a sinister smile.
Las nubes siniestras anunciaban una tormenta.
The sinister clouds foretold a storm.
In Spanish, 'siniestra' can also refer to the left side, but this meaning is less common and often replaced by 'izquierda'.
'Siniestra' is rarely used to mean 'left' in modern Spanish. Use 'izquierda' instead unless describing something as sinister.
From Latin 'sinister', meaning 'left' (associated with bad omens in ancient times).
While 'siniestra' can mean 'left' in some contexts, it is more commonly used to describe something as sinister or ominous. For the left side, 'izquierda' is preferred.