What kind of sentence structure is this? "Too little arousal and one is dull, ineffective."
I don't understand why We can grammatically put the word "ineffective" at the end of this sentence?
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The sentence is making use of coordinate adjectives.
With coordinate adjectives, all instances of and can be replaced by a comma. For instance, each of the following sentences can be written in various ways:
The heavy and bulky box.
The heavy, bulky box.
The tall and dark and handsome stranger.
The tall, dark, and handsome stranger.
The tall, dark, handsome stranger.
In both of these examples, all variations are acceptable. (In conjunctions with more than two items, the final and is commonly left in place—but it doesn't need to be in this case.)
In the second example, it would be less common to see the first variation than the others—but there's nothing that precludes its use.
Coordinate adjectives can also come after a noun:
The box is heavy and bulky.
The stranger is tall and dark and handsome.
But can the same replacement of and with commas be used here?
The box is heavy, bulky.
The stranger is tall, dark, handsome.
One is dull, ineffective.
I can find no rule that prohibits this, although it is uncommon and can look "strange" because it's something that isn't normally seen.
However, newspaper headlines (particularly in the U.S.) do sometimes replace such instances of and with commas—theoretically to save space; however, I'm not certain if it still makes sense or if saving space was why the practice originated but it's only continued out of habit.
This practice is sometimes referred to as a "headline comma" or, more generally, "headlinese," and you'll find a lot of discussion on it if you search for either of those terms online.
An example of such a headline is:
So, while I believe that the sentence you gave is not actually ungrammatical, it's using an uncommon style that would typically be rephrased.