lose one's temper

Example: “He lost his temper when the printer jammed for the fourth time.”

Origin: "Temper" traces through Latin temperare, to mix or moderate, originally describing a properly balanced mental or physical state rather than anger specifically — the same root that gives English "temperature" and "temperate." A person with good "temper" was even-keeled and well-regulated; "losing" that temper meant losing the internal balance holding emotional reactions in check, tipping over into the kind of visible, uncontrolled anger the phrase now names almost exclusively. The specific idiom in this narrowed, anger-focused sense is documented from the early 1800s, by which point "temper" alone had already drifted from meaning "balanced disposition" generally toward meaning, more often than not, the anger that results when that balance fails.