put on armor, arm oneself
鎧 centers on the act of donning protective metal covering: putting on armor or arming oneself for battle.
鎧 combines 金 (metal) with 豈, which likely contributes the sound and may relate to a sense of covering or protection. The exact historical development of 豈 in this context is uncertain.
The left side 金 is metal, and the right side 豈 looks like a helmet with a visor and a body plate. Picture a warrior putting on this metal armor piece by piece.
For カイ, imagine a knight in shining armor shouting 'Kai!' as he arms himself: Kai -> カイ.
armor; armour
box-shaped heavy armour, esp. used on horseback; ō-yoroi
plate armor; plate armour
heavy armor; heavy armour; wearing heavy armor
to put on armour (armor); to wear armour; to arm oneself
giant burrowing cockroach (Macropanesthia rhinoceros)
slatted shutter; louver door (window)
louver slat; louvre slat
storage chest for a suit of armour and a helmet
American football
louvered window
ankylosaur (any dinosaur of infraorder Ankylosauria)
Angelica dahurica (species of angelica)
girdle-tailed lizard
(beating someone) hands down; with a single blow
kitefin shark (Dalatias licha); seal shark
armadillo
snubnose brotula; armoured cusk (Hoplobrotula armata)
leather armor; leather armour
horse armor; horse armour