early 15c., "impetuous, rash," altered from Middle English testif "headstrong" (late 14c.), from Anglo-French testif, Old French testu (Modern French têtu) "stubborn, headstrong, obstinate," literally "heady," from teste "head" (see tete). Meaning "easily irritated, irascible" is first recorded 1520s. Related: Testily; testiness.