cover over, hide, narrow-necked
弇 centers on covering or concealing something, often by narrowing or enclosing it. The idea of a narrow neck extends this to a physical shape that hides or restricts what is inside.
The exact historical development of 弇 is uncertain. It visually combines 合 (fit, join) and 廾 (two hands), which may suggest hands covering or enclosing something, but this is speculative.
The top 合 suggests joining or fitting together, and the bottom 廾 looks like two hands. Imagine two hands cupping over an opening to cover it completely, like hiding something in a narrow-necked jar.
For エン, picture the hands covering the opening of an envelope (エン). The envelope hides the letter inside, just as 弇 means to cover or hide.