in other words, thou, you, possessive particle
廼 is an archaic character with a cluster of grammatical and pronominal meanings: it can mean 'in other words' or 'that is', act as a possessive particle, or serve as an old second-person pronoun 'thou'/'you'. These uses do not share a single concrete core idea, so treat it as a multi-purpose classical function word.
廼 is a variant form of 迺, which historically combined the movement component 廴 (or ⻌) with 西, possibly used phonetically to represent a grammatical word. The exact origin is uncertain, and the character is now rare.
The enclosure 廴 suggests movement or extension, while 西 (west) sits inside. Imagine an old-fashioned way of pointing westward and saying 'that is the west' or 'you, over there in the west' — capturing the archaic 'in other words' and 'thou' senses.
For ナイ, picture a knight in archaic speech pointing westward and declaring 'Nay, I say unto thee!' — the 'nay' (ナイ) sound fits the old-fashioned pronoun and particle use.