It can be an adjective:
* "That was a crazy idea!" (Describing the idea)
* "She's a crazy cat lady." (Describing the person)
It can be an adverb:
* "He was driving crazy fast." (Describing how fast he was driving)
* "The music was playing crazy loud." (Describing how loud the music was)
It can be a noun:
* "Don't go crazy on me." (Meaning: don't become irrational or act out)
* "The kids were going crazy." (Meaning: they were behaving wildly)
It's important to note:
* "Crazy" is often used in informal settings. In formal writing, you might want to use more precise words to describe the idea, person, or action.
So, to answer your question:
On its own, "Crazy" is not a complete sentence. It needs context and other words to be grammatically correct.