'arachnidan' originates from New Latin, specifically the word 'Arachnida', where the Greek root 'arachne' meant 'spider'.

'arachnidan' was formed in English from New Latin 'Arachnida' (used in biological classification); the New Latin term itself was taken from Greek 'aráchnē' and was adopted into scientific English to refer to the class of spider-like animals.

Initially it referred in scientific New Latin to the group of spider-like animals; in English it has been used to mean either an individual member of that group or an adjective describing things related to that group.