Cas is a nickname for Castiel, first used by Dean in 4.04 Metamorphosis when he says to Sam "Cas said that if I don't stop you, he will." From season five it is commonly used by other characters. Before its use in canon there were relatively few instances of the use of this diminutive in fandom, and when it did appear it was generally pilloried. Subsequently its use became widespread.
It was revealed he show's writers spell the nickname "Cass" in scripts, and it also appears that way in Tie-in novels and the Official Companion books. It is also spelled Cass in the season seven promo, the promo for the sixth episode of season eight and also for the promo of the eighteenth episode of season nine. On the Season 4 DVD the English subtitles call him "Cass" and other times it is "Cas."
In fandom, and particularly in fanfiction, it is almost exclusively spelled "Cas." Fans often get annoyed at the show's writers for spelling it differently. One theory as to the spelling "Cass" is that the writers based Castiel on the angel Cassiel, best known in popular culture from the movies Wings of Desire and the American remake City of Angels.
However a question from a curious "fan" to creator Eric Kripke on Twitter prompted a much simpler answer:
- Robbie Thompson: why is Castiel’s nickname spelled Cass not Cas? again: for science
- Eric Kripke: I think “Cass” just looks cooler on the page. And “Cas” might sound like “Caz.” But that’s just my opinion. Now get back to work.
During 12.01 Keep Calm and Carry On in October 2016, Misha sent a tweet out with "Cass" in it (despite previously having spelled it Cas himself), which fandom enthusiastically corrected for him. This lead to Misha doing a live stream the next day, calling for the opening of negotiations on a consensus spelling.
Despite this, some fans continue to insist that Cas is the only acceptable spelling.
- Cassiel in Judeo-Christian lore.