'equal-sided' is a compound formed in English from 'equal' + 'sided'. 'equal' originates ultimately from Latin 'aequalis', where 'aequus' meant 'level, even'. 'side' originates from Old English 'sīde', meaning 'side'.

'equal' passed into English via Old French/Latin influence (Latin 'aequalis' → Old French/Anglo-Norman forms → Middle English 'equel'/'equal'), while 'side' comes from Old English 'sīde'; the compound 'equal-sided' is a modern English formation combining these elements to mean 'having equal sides'.

The components originally meant 'even/level' (aequus) and 'side'; combined, they have consistently come to mean 'having sides that are equal in length', with little semantic drift.