Not to be thought of can be replaced with various synonyms depending on the context and situation. Some of the suitable options include unthinkable, unimaginable, inconceivable, absurd, out of the question, impossible, preposterous, beyond belief, beyond the pale, and far-fetched. These synonyms convey the idea that the subject under discussion is not worth considering or is so unlikely that it should not be entertained. Not to be thought of is an idiomatic expression used when something is considered improper, inappropriate, or simply implausible. Using synonyms can help to add variety and express the same idea differently, making the language more nuanced and precise.