He was indifferent to her feelings, which hurt her deeply.

indifferent 是由前缀 in-（表示"不"）和 different（"不同的"）组成，字面意思是"没有区别的"，引申为"漠不关心的""冷淡的"。这个词强调对事物缺乏兴趣或关注的态度。