'even-tempered' originates from English, specifically the combination of the words 'even' and 'temper' with the suffix '-ed', where 'even' meant 'level, equal' and 'temper' referred to 'disposition or temperament'.

'temper' originates from Latin, specifically the word 'temperare', where 'temper-' meant 'to mix in proportion, to moderate'. 'temperare' passed into Old French (e.g. 'temperer') and Middle English as 'temper', and the compound 'even-tempered' developed in modern English by combining 'even' + 'temper' + '-ed'.

Initially 'temperare' meant 'to mix or moderate', and 'temper' in older English carried senses of moderation; over time 'temper' shifted toward the sense of 'disposition, mood'. Consequently, 'even-tempered' came to mean 'having a steady, moderate (i.e. calm) disposition'.