long
縯 centers on the idea of something drawn out or extended in space, giving the sense of long. The thread component reinforces the image of a long, continuous strand.
縯 combines 糸 (thread) with 寅, which likely contributes the sound and may relate to extension or drawing out. The exact historical development is uncertain.
The left side 糸 is a long thread, and the right side 寅 looks like a stretched-out shape. Together they suggest something long and extended, like a thread pulled taut.
For エン, imagine an endless long thread: endless -> エン, and the thread stretches on and on.