The asperity in his tone when he replied took me by surprise.

asperity 是名词，来自形容词 asper（粗糙的、严厉的），加上名词后缀 -ity 构成。这个词本义指"粗糙、严酷"，引申为"（语气或态度的）严厉、尖刻"。在句中形容说话时语气尖锐刺耳。