A colloquial conjunction meaning 'despite' or 'and yet', often used critically or with annoyance (similar to 'kuse ni').
仕事ができないくせして偉そうにする。
He acts arrogant even though he can't do the work.
学生のくせして授業をサボるなんて信じられない。
I can't believe a student would skip class like that.
偉そうなくせして、自分では何もしない。
Although he talks big, he does nothing himself.