He stirred his coffee absently, not noticing the milk had already filled the cup.

absently 是形容词 absent（缺席的、不在的）加上后缀 -ly 构成的副词形式。absent 本意是"缺席"，引申为"心不在焉的、走神的"。absently 表示"心不在焉地、漫不经心地"，描述一种注意力不集中的状态。