means Temper is your habitual mood or emotional balance, the heat of anger you can lose or keep, and also the act of strengthening or moderating something — like steel or chocolate — by carefully controlling its heat.

from From Latin temperare, 'to mix in due proportion, regulate, moderate' — the same impulse behind 'temperature' and 'temperate.' English took it in via Old English temprian, first about mixing and moderating things, then about the blend of qualities (the four humors) that set a person's disposition. From that 'mixture of the self' came our 'temper' as mood, and the metalworker's 'temper' as the precise heat-and-cool that gives a blade its hardness. The bad-tempered sense — flying into a rage — is a later narrowing, as if 'temper' shrank to mean only the heat that's lost rather than the balance that holds it.