This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 乁 is uncertain. It is a rare character with a single stroke and a dot, but its origin is not well documented.
The shape resembles a flowing line with a drop at the end, like a stream flowing and then dripping. This can suggest the idea of flowing, but the meaning is not firmly established.
For イ, imagine the single stroke as a flowing river that ends with a drop, and think of the English word 'e' (as in the letter E) flowing into the shape: 'e' -> イ.