You're probably going "What are you talking about?" at this point. Indeed, it makes no sense even once you know what it is.
If you look at Hopper's Annotated Map of Ok, Who Else Wants Some?, you'll see some odd text at both the top and bottom of the map. The top text looks like "1993", and the bottom text looks like "RSNAIL". Thus, "1993 RSNAIL" or "RSNAIL 1993". Yeah, doesn't make a whole lotta sense does it?
Now, me being a member of Club Penguin (an MMORPG for kids), I know that Rsnail is an abbreviation of RocketSnail, the game company that I think originally made Club Penguin. Oddly, RocketSnail was created in 1999, long after PiD was made.
It just gets weirder and weirder, doesn't it? ;)