from, possessive particle, whereupon, accordingly
乃 is a classical grammatical character that marks possession, origin, or a sequential link: it functions like 'from', 'of', or 'whereupon'. Its meanings are all abstract relational particles, not concrete objects or actions.
乃 is an ancient pictograph whose original concrete depiction is uncertain. It was borrowed early as a grammatical particle for possession and sequence.
Picture 乃 as a curved hook pulling something from one place to another, like a possessive link or a 'from' arrow. Its simple shape suggests a connecting function.
For ナイ, imagine a knight's possessive claim: 'This sword is from the knight!' Knight -> ナイ, and the knight's curved hook (乃) marks possession.