Siwan is not a word found in most English language dictionaries, and therefore it does not have any established synonyms in the language. However, it is possible that Siwan may be a proper noun referring to a place, person, or a particular event. In such cases, other terms that could be used to refer to the same entity may include alternative spellings or variations of the name. Additionally, a thesaurus may provide alternative terms that can be used interchangeably with a different word that shares some similarities in meaning, but that would depend on the context and intended usage.