lane, alley
衖 centers on a narrow passageway in a built-up area: a lane or alley. Both given meanings point to the same idea of a small side street.
衖 combines 行 (going, road) with 共, which likely contributes a phonetic element. The character is used for a lane or alley, fitting the road-related meaning of 行.
The outer 行 suggests a road or path, and the inner 共 looks like two hands holding something together. Picture a narrow lane where buildings crowd together, forming a tight alley.
For コウ, imagine walking down a narrow lane and hearing a cow mooing from behind a fence: cow -> コウ, and the sound echoes through the alley.