What is the meaning of 甲?

carapace, shell

something A

1. first in rank
2. instep or the back of the hand
3. the first party

first in rank

instep or the back of the hand

the first party

the first of the ten heavenly stems

armor

treble range

the first of the ten heavenly stems

number one, thus the best

something A; the first party

Source: wiktionary.org