The rain fell incessantly all day, making everyone feel irritable.

incessantly 是由形容词 incessant（不停的、持续不断的）加上后缀 -ly 构成的副词形式。incessant 本身由前缀 in-（表示"不"）和拉丁词根 cessare（停止）组成，字面意思是"不停止的"。incessantly 表示"不停地、持续不断地"。