'arachnoidal' originates from New Latin/Modern Latin 'arachnoideus', ultimately from Ancient Greek 'arachnē' meaning 'spider' and the Greek-derived suffix '-oid' from 'eidos' meaning 'form' or 'likeness'.

'arachnoidal' developed from the English adjective 'arachnoid' (from Greek 'arachnē' + '-oid') with the addition of the adjectival suffix '-al' (from Latin/Medieval Latin) to form 'arachnoidal'; the anatomical sense (relating to the arachnoid mater) became established in medical English.

Initially it carried the sense 'spider-like' (from Greek 'arachnē' meaning 'spider'); over time it acquired the specialized anatomical meaning 'of or relating to the arachnoid mater' while still retaining the descriptive sense 'web-like' in nonmedical contexts.