expression of surprise
咦 represents a vocal expression of surprise, similar to an exclamation like "eh?" or "huh?" in English.
咦 combines 口 (mouth) with 夷, which likely contributes the sound. The character is used to represent a surprised vocal sound.
The mouth radical 口 beside 夷 suggests a sound coming from the mouth. Imagine someone opening their mouth wide in surprise, making an "eh?" sound.
For イ, picture someone so surprised they can only say "E!" — the single sound イ matches the exclamation.