A conjunction/particle used to express 'and yet' or 'even though' with a critical or resentful nuance; indicates contrast between a fact and an unexpected or displeasing result.
彼は学生のくせに、漢字が全然読めない。
Even though he's a student, he can't read kanji at all.
お金がないくせに、高い服を買うなんて信じられない。
I can't believe you'd buy expensive clothes when you don't have money.
親のくせに子どもの気持ちを考えないのはひどい。
It's awful for a parent not to consider a child's feelings.
あの人は経験者のくせに、基本を忘れている。
Even though that person is experienced, they've forgotten the basics.