I know that S rank/class is above the standard A-F ranks/classes. A-F are used because they're the first letters of the alphabet. Is there a reason S is used, and/or does it stand for something? Maybe super or superior?
There's really no mention about what it stands, as far as I can recall. In fact, S-rank is not only used in One Punch Man. Reading around some articles, it seems like most of them attribute the S-rank as having originated in Japan. The meaning of 'S', however, is still unknown and is subject to speculation.
From what I've read, however, it seems like there are two possibilities as to where it originated:
- Japanese video games - there is a question about that in Arqade, with articles saying that it originated from Japanese videogames but it lacks citation, or rather, the sources the answer linked has no citations or references. A Reddit thread from a Japanese, however, provide games and manga from the 90's which uses S-rank. There is even mention of how concert seating ticket uses S-rank, too.
Now (2017), in Japan concert tickets are normally classified into "S席(=seat)" > "A席" > "B席". I googled and at least it had been used in 1972.
- Japanese grading system - from the Reddit thread linked above, it was also mentioned that it's possible that it came from an early class ranking system in the 1910's. To quote:
I think "S席" may mean "Special Seat". In Japan at least 1910, "特等" grade was used for the top grade seats (http://parasiteeve2.blog65.fc2.com/blog-entry-1135.html). "特等" > "一等(first grade)" > "ニ等(second grade)", or "特等" > "普通(normal or basic)" were used past (these are also commonly used for seats grades even now). "特等" is abbreviation for "特別等級" = "Special Grade". So S may mean Special.
Other board discussions links school websites and other forums with users going to universities or schools which uses up to the S-rank grading systems (Source 1, Source 2) There is also a Wikipedia article that shows that S-rank is used, although rarely, in schools.
Most discussion about what the 'S' means are speculations, and they seem to center on the fact it could mean 'special' or 'super'.
As for One Punch Man, there is no official information as to what it means in Saitama's world, either. Based on what I was able to find, though, I think it does originate in the ranking system present in Japan and the mangaka might have based this one on that.