'chiasmus' originates from Greek, specifically the word 'chiastós' (χιαστός), where the element related to 'chi' (the Greek letter χ) conveyed the idea of 'crossed' or 'X-shaped'.

'chiasmus' was borrowed into Latin and then into English from Greek (via Latin or directly from Greek usage); the plural 'chiasmi' follows a Latin-style pluralization of the Greek/Latin form and has been used in English scholarly and rhetorical contexts.

Initially it meant 'a crossing' or 'X-shaped arrangement' in Greek, and over time it came to denote the rhetorical pattern of reversing words or structures — the modern meaning remains closely tied to that image of crossing or inversion.