'arthritides' ultimately derives from New Latin/medical formation based on Greek: 'arthr-' from Greek 'arthron' meaning 'joint' and the suffix '-itis' (Greek '-itís') meaning 'inflammation'; the plural uses a Greek-style ending.

'arthritis' entered English from Modern Latin/Medieval Latin 'arthrītis' (from Greek 'arthritís'); the plural 'arthritides' reflects a Greek plural formation (akin to changing '-is' to '-ides').

Initially it referred medically to 'inflammation(s) of the joint(s)' in Greek/Latin medical usage; today 'arthritides' is simply the plural form used in English for multiple instances of arthritis.