Well-nigh means almost or very nearly. When something is well-nigh impossible, it's so difficult that it comes close to being impossible, though perhaps not quite. If a task seems well-nigh complete, you're almost finished with just a tiny bit left to do.

This word appears often in older books and formal writing. You might read that a castle was “well-nigh impregnable,” meaning it was nearly impossible to attack successfully. Or a character might say a journey was “well-nigh unbearable” because it was almost too difficult to endure.

The phrase combines “well” (meaning thoroughly or completely) with “nigh” (an old word for near). Together they create the sense of being very close to something without quite reaching it. Today, people more commonly say “almost” or “nearly,” but well-nigh adds a touch of emphasis or old-fashioned flavor. When you encounter it in your reading, you can usually replace it mentally with “almost” and the sentence will make perfect sense.