When you do something testily, you're not just mad — you're easily irritated or frustrated. If you respond testily to your little sister's request for more ice cream, you're probably fed up with hearing her voice. When people are tired or overworked, they're more likely to speak or act testily. The origin is Middle English, testif, "headstrong," from the Old French testu, which means "stubborn," or literally, "heady," with its Latin root testa, or "skull."