This puzzle is in the spirit of the What is a Word/Phrase™ series started by JLee with a special brand of Phrase™ and Word™ puzzles.
I also want to thank a math teacher who shared this riddle when I was in a high school. This riddle is a variation.
For each number from 1 to 9, there exist corresponding K-words™. Here are examples and non-examples:
$$ \begin{array}{l|l|l} & \text{K-words™} & \text{Not K-words™} \\ \hline \text{1st} & \text{Japan} & \text{Korea} \\ \text{2nd} & \text{Yttrium} & \text{Scandium} \\ \text{3rd} & \text{Uncle} & \text{Father} \\ \text{4th} & \text{Deception} & \text{Violence} \\ \text{5th} & \text{Duck} & \text{Chicken} \\ \text{6th} & \text{Army} & \text{Navy} \\ \text{7th} & \text{Blackboard} & \text{Chalk} \\ \text{8th} & \text{Arm} & \text{Leg} \end{array} $$
The objective is to give an example of a 9th K-word™.
For anyone who need it, here is a CSV version:
1st Japan Korea
2nd Yttrium Scandium
3rd Uncle Father
4th Deception Violence
5th Duck Chicken
6th Army Navy
7th Blackboard Chalk
8th Arm Leg
(Though, you don't need a CSV version. A dictionary suffices.)
Subtle Hint:
Other examples and non-examples of K-words™:
K-words Not K-words 1st Solar eclipse Lunar eclipse 2nd Tooth Tongue 3rd Samsung Apple 4th Human Alien 5th Error Success 6th Carnivore Herbivore 7th Chile Argentina 8th Octagon Circle
Moderate Hint:
Somehow, K-words™ start with their assigned number.
Decisive Hint:
One of the non-examples is a crucial clue.