The phrase "such a thing" refers to a specific item, concept, or event that the speaker is referring to. There are many synonyms for this phrase depending on context, tone, and intention. Some of these synonyms include "a particular item," "a specific matter," "an occurrence," "a situation," "an event," "a circumstance," "a phenomenon," "an incident," and "a happening." Each of these synonyms carries a unique connotation and can be used interchangeably with "such a thing" to provide more detail and specificity to the speaker's message. In order to choose the best synonym to use, it is important to consider the context in which it is being used and what impression the speaker wants to convey.