This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The historical origin of 巸 is uncertain. It visually contains 巳 on the right and a left side that resembles 臣, but the exact development is unclear.
Since the meaning is unclear, focus on the shape: the left side looks like a big chin (臣 can suggest a chin), and the right side is 巳 (snake). Imagine a snake resting on a large chin.
For イ, picture an eagle (イーグル) perched on the big chin shape: eagle -> イ, and the eagle's beak rests on the chin.