apathetic
Not caring or showing interest, even when you should.
Apathetic means not caring about something, showing no interest or emotion even when you probably should. An apathetic student might shrug when asked about their failing grade, showing no concern about improving. An apathetic voter doesn't bother learning about candidates or issues because politics doesn't interest them.
The word describes a lack of feeling that goes beyond simply being calm or patient. When your friend is apathetic about which movie to watch, they genuinely don't care either way. When a team becomes apathetic after losing several games in a row, they stop trying hard because they've lost their motivation and spirit.
Apathy is the noun form. You might hear someone complain about the apathy of their classmates when nobody volunteers to help with the school fundraiser. Teachers sometimes worry when they see apathy spreading through a classroom, with students slumping in their chairs and staring blankly instead of engaging in the lesson.
Being apathetic is different from being thoughtful or reserved. A thoughtful person cares deeply but takes time to respond. An apathetic person simply doesn't feel invested in the outcome.