gatekeeper, gate, door
閽 centers on a person who controls access through a gate or door: a gatekeeper. The gate/door sense is the physical barrier they guard.
閽 combines 門 (gate) with 昏, which likely contributes the sound and may relate to dusk or dimness, perhaps suggesting a gatekeeper working at twilight. The exact historical development is uncertain.
Outside the gate 門, a dim 昏 figure stands guard. The gatekeeper controls who enters when light is low.
For コン, imagine a con artist trying to talk their way past the gatekeeper: con -> コン, and the gatekeeper must stay alert.