Is witless and occur an synonym?

No, "witless" and "occur" are not synonyms. They have completely different meanings:

* Witless: Means lacking intelligence or common sense; foolish.

* Occur: Means to happen or take place.

Here are some examples:

* Witless: "The witless criminal left his fingerprints at the crime scene."

* Occur: "The meeting will occur tomorrow at 10am."