Deaccelerate means the same as decelerate, though it seems to be a much less common alternative. I did not know this until recently, as I had used this alternative all my life. It just seemed logical to me, deaccelerate is accelerate with a negative prefix. I came to believe that deaccelerate was the "father" of decelerate, as decelerate seems like a word that derived from the former word due to practicality. I do not know if this is true though. What I do know is that decelerate is a lot more used and much more popular than deaccelerate, something I am reminded off when my browser constantly puts a red line under the word. Yet, it is listed in many dictionaries, proving that it is indeed a valid word.
So then my question is, why isn't deacceleratingly a valid word? Both deaccelerating and deceleratingly are valid words, but deacceleratingly is not listed anywhere and also gets a red line under itself. It just doesn't make sense to me to leave out this piece of the "word's set". Of course, deaccelerate is unpopular, so it's safe to assume that deacceleratingly is even less used, but does it really take that much time to add the word into the dictionaries and Chrome's own browser? I know that this is getting a bit too discussion-like, so I'll return to my core question.
Is deacceleratingly a word? Is it simply ignored by all the dictionaries, but still technically a word, or is it a completely invalid and ungrammatical word?