It's important to note that there is no single perfect equivalent. While pessimism encompasses the general concept of negativity and a negative outlook, it doesn't fully capture all the nuances of the adjective "pessimistic". Other words that could be used to describe a pessimistic outlook include:
* Gloom
* Despondency
* Cynicism
* Skepticism
* Doubt
* Negative outlook
The best choice depends on the specific context and the meaning you want to convey.