A duck hook is a shot that curves hard to the left of your target, if you are a right-handed golfer. A duck hook almost always gets a golfer into some kind of trouble on the course.
A closed club face at impact and an inside-to-out swing can cause a duck hook. Learning to swing with a square club face, to release the club properly and to swing on the proper path will help eliminate your duck hook.
Keep your left arm straight as you swing to the top of the backswing. A straight left arm position helps you avoid an early release on the downswing. An early release occurs when you lose your wrist hinge and the club face closes before impact. Stop your backswing when your left shoulder is under your chin.