To underappreciate means to fail to recognize the full value or importance of something or someone. When you underappreciate a person's effort, you might say “thanks” without realizing how much work they actually put in. When you underappreciate a book, you might finish it and move on without noticing the clever way the author structured the story.

People often underappreciate things they see every day. You might underappreciate your school librarian until you need help finding a specific book and realize how much she knows about the entire collection. A coach might underappreciate a quiet player who does all the small things right, focusing instead on the loud star who scores the goals.

The word suggests a gap between reality and recognition. The thing being underappreciated has real worth, but that worth goes unnoticed or unacknowledged. Sometimes we underappreciate things until they're gone: you might not realize how much you valued playing outside until winter arrives, or how helpful a friend was until they move away.

The opposite is overappreciate, though that's less common. More often, we simply appreciate something fully, giving it the recognition and gratitude it deserves.