descriptive of creaking, laugh
咿 is a rare kanji used to describe sounds: a creaking noise or a particular kind of laugh. Its meaning is tied to vocal expression rather than a concrete object or action.
咿 combines 口 (mouth) with 伊, which likely contributes the sound イ. It was created to represent vocal sounds such as creaking or laughing.
The mouth radical 口 beside 伊 suggests a sound made with the mouth. Picture a door creaking open with a long 'eee' sound, or someone laughing with a high-pitched 'hee-hee'.
For イ, imagine the creaking sound as a long 'eee' (イ) coming from an old door. The mouth 口 is wide open, making that exact sound.