1. causing or involving suffering, fear or unhappiness
a dreadful mistake/error/time, dreadfully slow
Dreadful is an adjective describing something awful, unpleasant or which causes suffering in some way. Something 'dreadful' literally fills us full of dread. You can have a dreadful fight or accident for example. Of then this word is used colloquially to describe something that we want to emphasise is very bad. "This food is dreadful." You will hear this adjective used often in all kinds of contexts.