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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?

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    Do you have a source for "deceleratingly" as a word? This implies otherwise. Similar for your variant spelling, although ngrams implies a few hits (could be misuse, though). Decelerating is a word. – Pam Aug 15 '18 at 13:00
  • en.wiktionary.org/wiki/deceleratingly yourdictionary.com/deceleratingly wordnik.com/words/deceleratingly Please do not cite one dictionary and say that the word doesn't exist. – A. Kvåle Aug 15 '18 at 13:11
  • Are you asking about deceleratingly or deacceleratingly? Your comment points to the former but your question refers to the latter. Either way, English is a productive language. The prefix de- and the suffix -ly can be added to many words, with the resulting words not necessarily catalogued in dictionaries. – Lawrence Aug 15 '18 at 13:16
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    "Decheesingly" is a perfectly valid and cromulent word. You are able to explain its derivation and meaning. Perhaps you need to define what you mean by "valid word"? Do you mean one that someone else has invented? Or one that has been used more than once? Or often enough to get into one dictionary? Or ... ? – user184130 Aug 15 '18 at 13:46
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    Regarding deaccelerate vs decelerate, both go back to Latin celerare ‘hasten’, from the adverb celer meaning ‘swiftly’ (cf. celerity in English). Accelerate is ad- ‘towards’ + celerare ‘swiftness’, literally ‘going towards hastening’ = speeding up. Decelerate, then, substitutes ‘towards’ for ‘away from’: ‘going away from hastening’ = slowing down. Deaccelerate combines them, meaning ‘going away from going towards hastening’, which sort of ends up being the same thing, but is semantically much more complex. – Janus Bahs Jacquet Aug 16 '18 at 14:39
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You are correct, you can use words deaccelerate or deacceleratingly.

You won't find it in standard dictionaries like Merriam-Webster or Oxford. But you can certainly use them.

But, yes, it is true, I seldom came across the words called "deaccelerate" or “deacceleratingly" in any of the reference books while in my under-graduation in physics.

Also, deaccelerate is not a mother of decelerate. Actually, the verb decelerate(1899) is a backformation of the noun deceleration (1894).

You can use prefix de- to make combining forms e.g. deaccelerate or deacceleratingly:

  1. de- is added to a verb in order to change the meaning of the verb to its opposite.
  2. de- is added to a noun in order to make it a verb referring to the removal of the thing described by the noun.

So, yes you can use deaccelerate or deacceleratingly, it is not grammatically wrong.

  • The fact that "deaccelerate" is uncommon and rarely used was not new to me, as I mentioned in my question. That you didn't come across it either does not really answer my question as it only tells me your subjective experience of the word, and not a source saying whether the word is proper English or not. – A. Kvåle Aug 15 '18 at 13:14
  • @A.Kvåle you are not wrong. It is rarely used. I have edited further :) – Ubi hatt Aug 15 '18 at 13:17
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    Thank you, your edit improved much. Sorry if I was a bit douchey in my comment. Good that you added de-, as I didn't know a prefix could have an hyphen after it. – A. Kvåle Aug 15 '18 at 13:22
  • I think the second year should be 1899, not 1988, based on your reasoning and the link. – jejorda2 Aug 15 '18 at 13:38
  • @jejorda2 Oh! the typo. Thanks! edited :) – Ubi hatt Aug 15 '18 at 13:42
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The OP’s, very understandable, mistake was to think of accelerate as the ‘father’ of the word in question. This led the OP to add de- to it, analogously to the way in which we, for example, add de- to alcoholise to get dealcoholise, the verb for the opposite action. It is, however, better to think of decelerate and accelerate as siblings. The a- at the beginning of accelerate is itself a prefix; both words are thus formed by adding different prefixes to the common root. Their ultimate ancestor is the Latin word celer, meaning fast, swift, rapid; to accelerate is thus to make something fast, to decelerate is to make it the opposite of fast, that is slow. While, there is no word celerate in English, there is celerity which belongs to the same family, and makes the root more clearly visible. The relationship between accelerate and decelerate is analogous to that between appreciate and depreciate (note that we do not say deappreciate), not to that between alcoholise and dealcoholise.

Another way to think of it is this. What does the action of decelerating take away? It does not just take away acceleration: if one accelerates, and then merely takes away the acceleration, the result is that the motion continues at the speed, possibly very high, which was reached by the acceleration. The action of decelerating takes away the speed, velocity, celerity. That’s why the word is decelerate rather than deaccelerate.

Using the word deaccelerate is thus not merely a stylistic faux pas that would annoy some stuffy linguistic prescriptivists, but is otherwise harmless. It can create confusion, because when we hear it, we cannot be sure whether it is used as a variant of decelerate, or to mean removing the accelaration (but possibly continuing at high speed), which is what it would mean, according to the principles that generally govern the prefix de-.

  • Good point, the ending of acceleration and continuing to move at the final speed could well be called deaccelerating. However the ending of deceleration and continuing to move at the final speed would, presumably, be dedecelerating. – BoldBen Aug 16 '18 at 16:39

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