in other words, thou, you, possessive particle
迺 is an archaic character with several distinct uses: it can mean 'in other words' or 'that is', serve as an old second-person pronoun 'thou/you', or function as a possessive particle like の. These meanings do not share one clear conceptual center, so treat it as a multi-purpose classical word-carrier.
迺 combines the movement radical 辶 with 西, but the exact historical development is uncertain. It may have originated as a phonetic loan or a semantic extension, but the modern form does not clearly reflect its original function.
The movement radical 辶 suggests a path or way, and 西 (west) inside can be seen as pointing westward. Imagine an old text where '迺' is used to rephrase something: 'in other words, go west.' The same character can also address 'you' who are heading that way, or mark possession of the western path.
For ナイ, picture a knight (ナイト) in archaic language saying 'thou' and using a possessive particle: the knight's old-fashioned speech cues ナイ.