'assized' originates from Old French (via Anglo-Norman) and ultimately from Latin; specifically from Old French 'assise' and Latin 'assidere' (ad- + sedēre), where 'ad-' meant 'toward' and 'sedēre' meant 'to sit'.

'assized' developed from the Old French word 'assise' (meaning 'a sitting, session') which entered Middle English as 'assise'/'assize' and later produced the English verb 'assize' and its past participle 'assized'.

Initially it meant 'a sitting' or 'session'; over time it came to mean specifically a judicial session or an ordinance/assessment made at such a session, and the verb sense came to mean 'to judge, to fix by assize'.