casual short form: 근데 · starts a sentence
but, however; by the way
Example
이 옷이 예뻐요. 그런데 좀 비싸요.
These clothes are pretty. But they're a bit expensive.
Word parts
From 그러하다 (to be so) + -ㄴ데.
Good to know
Also used to change the topic: 'By the way…'. In speech it's often shortened to 근데.