The term ‘worstest’ is often used in casual conversations, but it is not considered a standard English word. The correct superlative form of ‘bad’ is ‘worst.’
In English, superlatives are typically formed by modifying the base adjective. For example, we say ‘good’ becomes ‘better’ and then ‘best,’ while ‘bad’ changes from ‘bad’ to ‘worse’ and then to ‘worst.’ Therefore, saying ‘worstest’ is a grammatical mistake.
Some people might use ‘worstest’ to emphasize that something is exceptionally bad, but it’s important to remember that in formal writing and speech, ‘worst’ is the correct and accepted term to use.