Testy

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late Middle English (in the sense ‘headstrong, impetuous’): from Anglo-Norman French testif, from Old French teste ‘head’, from Latin testa ‘shell’.

wiktionary

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Middle English testif(“headstrong”), from Old French testu, from teste(“head”) + -u. Compare modern French tête(“head”), têtu(“stubborn”).

etymonline

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testy (adj.)

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.