9th grade Spanish class.
Teacher had us all come up with Spanish nicknames to use in the class to get us acquainted with the language. I wanted Miguel, because my name's Michael. However, Micah Farmer comes before Michael Moore when alphabetizing last names, and he got Miguel. The teacher gave me one more day to come up with a name. I had it down to Caesar and Memo (which is a nickname for Guillermo), so I chose the second one because it started with M.
There's a guy who still calls me Memo to this day.
When I got online, I figured Memo would be a good screen name. However, it was usually taken already. So I went with MrMemo most of the time. (Remnants of those days are still seen in my AIM screen name and NeoPets username.)
I got on a chat room called United Empire of Planets and was Mr. Memo for a long while. However, I decided I needed something more Trekish for my name, and Admiral had the M and R in it in the same order, so I recolored my new name, Admiral Memo, to have the M R and Memo to be the same colors as the first version of my nick.
After that, it stuck and I've been using it everywhere.