Cas (or Cass) 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 on, it's often 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 later revealed that the show's writers spell the nickname as "Cass" in scripts. It also appears that way in Tie-in novels and the Official Companion books. It's also spelled "Cass" in the season seven promo, the promo for the sixth episode of season eight and the promo of the eighteenth episode of season nine. Confusingly, on the Season 4 DVD, the English subtitles use "Cass" and "Cas" interchangeably.
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 predominantly using "Cas" on twitter in the past. The fandom enthusiastically corrected him, prompting Misha to post a photo of a Season 12 episode script he was in the middle of filming, followed by a satirical livestream posted to his Facebook the next day, jokingly calling for the opening of negotiations on a consensus spelling.
This all proved to be a promotion for a new range of merchandise from Creation Stands Holiday range which included products with the slogan, "It's Cas not Cass".
In episode 9.18 Meta Fiction Metatron uses the spelling Cass as he is typing his prophecy, making the spelling canon. In episode 12.07 Rock Never Dies Dean gets a call from Castiel and his name in Dean's caller ID is spelled "Cass," continuing this spelling as canon.
Despite all of the canonical evidence to the contrary, many fans continue to insist that Cas is the only acceptable spelling. Because we are right.
- Cassiel in Judeo-Christian lore.