(Latin: testu, testum; earthen container, earthen pot; shell, skull)
2. Etymology: from Late Latin testa, "skull"; in Latin, "pot, shell".
To see the "head" in testy, it is necessary to look back at the Old French word testu, which is the source of testy. Testu (literally "heady") is derived from the Old French word teste, "head" (Modern French tête).
In English testy developed another sense, "aggressive, contentious"; which passed into the meaning now used as, "irritable".