As promised, a new entry today. We’ll learn a lesson about dictionaries this time.
At first, when you think about tsuide, what might cross your mind is that this word really looks like a verb; perhaps even the te-form of tsugu?
“Well, let’s consult a dictionary”, is the first step usually. Doing just that, we find in the 広辞苑第六版(Kōjien, 6th ed.)
(from the continuative form of [the verb] tsuidzu)
The next dictionary, 精選版日本国語大辞典(Nihon Kokugo Daijiten, concise version) reads:
(noun) possibly an abbreviated version of tsugi-te
Two different dictionaries, two different explanations, and they are incompatible. At least one of them must be wrong.
As for the Nihon Kokugo Daijiten, it’s noteworthy that the author(s) felt uncertain about it as well. Unless it’s a printing error and the entry should read の変化した語から, which would express no uncertainty. Let’s assume for now it’s not a mistake. If the word really comes from tsugi-te, this would be an instance of a mixed Japanese-Chinese reading, which is somewhat unusual; and tsugi-te itself would already be a shortening of tsugi-tē. It’s possible, but it doesn’t seem likely.
Yet, on the other hand, the alternative version feels equally strange. It postulates the word ?tsuidzu, which would be a strange verb indeed: To my knowledge, it would be first verb ending on the the voiced syllable づ dzu. This made me quite doubtful when I first read this explanation, but after some more research, I found an entry in the 大辞林(Daijirin), listing exactly this verb.
Considering the above, it’s my opinion that the etymology from tsuidzu bring order to, put into order is the correct explanation. Interestingly, the Daijirin also mentions that tsuidzu itself derives from 継ぎ-つ tsugi-tsu, which would lift the mystery of the ending syllable.
Moral of the story, never trust any one dictionary unconditionally, always cross-check with other dictionaries. There are enough monolingual dictionaries available online, and free of charge at that.