This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 毉 is uncertain. It visually combines 殹 and 巫, but the original semantic and phonetic roles are unclear.
The top 殹 suggests a medical or arrow-related idea, and the bottom 巫 is a shaman. Picture an ancient shaman performing a healing ritual, but remember the meaning is uncertain.
For イ, imagine an ancient shaman saying 'Eee!' during a healing chant: 'Eee' -> イ.