sunken trap
穽 centers on a hidden pit used as a trap: a sunken hole designed to catch someone or something by surprise.
穽 combines 穴 (hole) and 井 (well), suggesting a pit or well-like trap. The exact historical development is uncertain, but the component combination clearly points to a sunken trap.
Picture a hole (穴) over a well (井): a hidden pitfall trap. Someone steps over the well-like opening and falls into the sunken trap.
For セイ, imagine a trap so clever it seems to say 'say' nothing: say -> セイ, and the trap silently waits.
trap; pitfall