After hearing the same lecture for the fifth time, his weariness was clearly visible on his face.

weariness 是形容词 weary（疲倦的）加上名词后缀 -ness 构成的名词形式。weary 表示"疲倦的、厌倦的"，后缀 -ness 表示状态或性质，所以 weariness 指的是"疲倦、厌倦的状态"。