be, particle, sigh, alas
繄 is a rare classical character whose meanings are fragmented: it can function as a grammatical particle, express a sigh or exclamation (alas), or simply mean 'be'. These uses do not share a single clear conceptual center.
繄 combines 殹 (a phonetic component) with 糸 (silk/thread). The exact historical development is uncertain, but it likely originated as a phono-semantic compound where 殹 contributed the sound and 糸 added a semantic nuance, possibly related to binding or continuity, which later extended to grammatical and exclamatory uses.
The top 殹 looks like a combination of 医 (doctor) and 殳 (action), while the bottom 糸 is thread. Imagine a doctor using thread to stitch a wound, and with each stitch they sigh 'alas' — the thread binds the action to the sigh, and the whole scene just 'is'.
For エイ, picture the doctor sighing 'Ayyy' as they stitch: 'Ayyy' sounds like エイ, and the thread (糸) ties the sound to the character.