If you shudder, you shake with fear, horror, or disgust, or because you are cold.

If something such as a machine or vehicle shudders, it shakes suddenly and violently.

If something sends a shudder or shudders through a group of people, it makes them worried or afraid.

If you say that you shudder to think what would happen in a particular situation, you mean that you expect it to be so bad that you do not really want to think about it.