The batter couldn't predict the path of the pitcher's knuckler as it danced unpredictably.

knuckler 是由名词 knuckle（指关节）加上后缀 -er 构成的名词，指"用指关节投出的球"。在棒球运动中特指"指关节球"，这种球因为投球方式特殊（用手指关节发力）会导致球路飘忽不定，难以预测。