The name of a fictional river doesn't count . But I guess it's obscure and creative. but also like why X? It's so overhyped. And the words we can use are so limited but also well-known. Like heck. Maybe I wanna be creative? But like if this was scrabble and you got points for giving people more obscure and tricky letters, you'dve made a cool play. Though there are other words starting with s and ending with x that are well-known so it's not like a clever play made out of necessity. oh well, because proper nouns are allowed: I play a reverse card and send your river Styx back to the graveyard: Xerxes
Deodorant You had nothing to say when I used the name of a fictional Chinese submarine a few pages back
Gravitational Did you know that there are no words in English that end with the letter V? Check this thread. No words end or start with V.
Go Improv is just short for improvise in which provise is similar to the word provide. So to improvise is to be without provisions. Not provided with anything(like a script). Unprepared. The ise isn't a suffix to improv. The im is the prefix to provise.