Shaitan was originally the Fremen term for a powerful embodiment of evil, such as the devil or a demon. Not surprisingly the term was usually spoken in a tone of fear and disgust. To a lesser extent, it was also a name given to the sandworms of Arrakis, certainly less so than Shai-Hulud.
Later the term was adopted by the Rakian Priesthood to describe the transformation of Leto Atreides II into sandworms. In this context the term became tied to the concept of the Divided God, and was used with reverence.