'arachnological' originates from Modern English, formed from 'arachnology' (the study of spiders), where 'arachn-' comes from Greek 'aráchnē' meaning 'spider' and '-logical' from Greek '-logia'/'-logos' meaning 'study' or 'word'.

'arachnological' developed from the noun 'arachnology' (coined in the 19th century in English) which itself was formed from Greek 'aráchnē' via New Latin/Modern formation; the adjective was formed by adding the English adjectival suffix '-al'/'-ical' to the noun.

Initially formed to denote 'pertaining to the study of spiders,' its usage has remained consistent and still means 'relating to spiders or arachnology.'