'arthritical' is a modern English formation modeled on 'arthritic' combined with the adjectival suffix '-al'; the combining form 'arthr-' comes from Greek 'arthron' meaning 'joint'.

'arthritic' entered English via New Latin/Modern Latin from Greek 'arthron' ('joint') + the adjective-forming suffix '-itic'; 'arthritical' is a later analogical formation built from 'arthritic' + '-al', rather than an established historical form.

Originally the Greek root 'arthron' referred simply to 'joint'; over time derivatives like 'arthritic' and analogical forms such as 'arthritical' came to mean 'relating to or affected by arthritis', a medical condition of the joints.