This is the 6158th most frequent Turkish word.
zehir
Poison.
Here, 'zehir' refers to a poisonous substance, specifically venom in this context.
Zehir bir yılanın avlanma silahıdır.
Venom is the hunting weapon of a snake.
In this sentence, 'zehir' is used metaphorically to describe the harmful effect of negative thoughts.
Kötü düşünceler insana zehir gibi gelir.
Negative thoughts feel like poison to a person.
Here, 'zehir' is employed in a colloquial manner to emphasize the spiciness of food.