If you look at "umop apisdn" upside down, you will see that it actually says "upside down".
An alternate spelling, "umop ap!sdn," fixes the upside down i problem with it, but makes the i lower than it should be when you look at it upside down. It's up to you which one you prefer.
Turning Text Upside Down has a Perl program that will turn any text upside down.