Let us examine its evolution history first: (image taken from http://vividict.com/)
|Learn one Chinese Character a day - 甲|
Let us check its next variation in Bronze Script - . It does provide us more information. First the changed to with a open mouth, and vertical leg ofsticks out of the open mouth, downwards (that is why I suspect that 甲 and 七 used to be same character in Oracle Script period with meaning of cutting through). We have just learnt in our last lesson that can have the meaning of grass. Now have we collected enough information to reason its meaning? The answer is yes. Let us see a picture below which reflects the vivid view of what represents:
So original meaning of 甲 is a seed sprouting with its coat opening up.
Eventually 甲 is used to refer to the hard coating of the seed. Then it is extended to mean hard skins of animals or scales of fish and reptiles. And it is also extended to mean something like nail. Because of the protection function of seed coat, shell, scale or nail, 甲 is also extended to mean equipments which protect human beings, like armour.
Do not forget the original meaning of 甲 as a seed starts to sprout that you would not feel surprised to know 甲 contains the meaning of being first place / best. And because of that, 甲 is also extended to mean first one in ten Celestial Stems (https://en.wikipedia.org/wiki/Celestial_stem). It is all because 甲 has the meaning of something starts to happen.
Now we have full understanding of the meaning of 甲, it is time to enjoy a Chinese calligraphy with 甲 inside: