'arachnida' originates from New Latin, specifically the word 'Arachnida', where Greek 'aráchnē' meant 'spider' and the suffix '-ida' was used in scientific Latin to denote a group or class.

'arachnida' changed from the Greek word 'aráchnē' (ἀράχνη) meaning 'spider', was Latinized in scientific usage as 'Arachnida' in New Latin, and eventually entered English as the taxonomic name 'arachnida'.

Initially it referred directly to 'spider' (the Greek root), but over time it evolved into a scientific term for the broader class that includes spiders and related animals.