Here's why:
* "M" is rarely silent: In English, the letter "M" is typically pronounced, even when it's followed by a silent "B" (like "dumb" or "climb").
* "N" is rarely silent at the end of a word: The letter "N" is generally pronounced at the end of a word. There are a few exceptions like "autumn" and "solemn," but these are not followed by "M".
Therefore, a combination of "MN" with both letters silent is not possible in standard English words.