'arachnidologist' originates from modern English, specifically the combination of 'arachnid' and the suffix '-ologist', where 'arachnid' meant 'spider' and '-ologist' meant 'one who studies'.

'arachnid' ultimately comes from Greek 'aráchnē' meaning 'spider', passed into New Latin as 'Arachnida' (the class name) and entered English usage in the 19th century; the element '-ologist' derives from Greek 'logos' ('study, word') via Latin/Old French into English as the productive suffix '-ologist', and the compound 'arachnidologist' was formed by combining these elements.

Initially it meant 'a person who studies spiders or arachnids', and this core meaning has remained stable into modern usage.