心不在焉 ---------- be absent-minded

例句：
A: How come he is often losing his things, such as books, keys, money?
他为何经常遗失东西，比如像书、钥匙、钱？
B: He is rather absent-minded all along.
他一向相当心不在焉。

同义词：
be wool-gathering
be day-dreaming
jump the track