'non-equilateral' originates from English by prefixing 'non-' (from Latin 'non', meaning 'not') to 'equilateral', which itself originates from Latin 'aequilateralis' where 'aequi-' meant 'equal' and 'latus' meant 'side'.

'equilateral' changed from Latin 'aequilateralis' to Old French/Medieval forms and then into modern English as 'equilateral'; 'non-equilateral' was formed in English by adding the negative prefix 'non-' to 'equilateral'.

Initially, 'equilateral' meant 'equal-sided'; adding the prefix 'non-' created the negated sense 'not equal-sided', and this negative meaning has remained consistent.