'coequal' originates from Latin, specifically the prefix 'com-' (variant 'co-') meaning 'together' and the word 'aequalis' meaning 'level, equal'.

'coequal' changed from the Old French/Latin word 'aequalis' through Middle English forms like 'equal' and was later formed in English by combining the Latin prefix 'co-' with 'equal' to create 'coequal'.

Initially, it meant 'together equal' or 'equal in rank', and over time it has retained and narrowed to the modern sense 'having equal authority, rank, or status'.