The definition for diffident is "Lacking self–confidence." Used in a sentence: The diffident student never raised her hand in class, even when she knew the answer.

The definition for dubious is "Questionable." Used in a sentence: Karl found Kevin's claim that he was sick dubious, as he had just seen him playing football.

The definition for ecstatic is "Overwhelmingly emotional." Used in a sentence: They were ecstatic when their team won the championship game in the last second.

The definition for flaw is "An imperfection." Used in a sentence: The otherwise perfect diamond had a tiny flaw that reduced its value.

The definition for preclude is "To rule out in advance." Used in a sentence: "Having cake now will preclude dessert later," said Amber's mother.