pitfall, trap, snare
阱 centers on a hidden danger that catches someone off guard: a pitfall, trap, or snare. The idea is a concealed hole or device that ensnares.
阱 combines the mound radical ⻖ (阜, suggesting earth or a pit) with 井 (a well or pit), together indicating a pitfall or trap dug into the ground.
The left side ⻖ suggests a mound of earth, and the right side 井 looks like a well or a pit. Picture a hidden pit dug into the ground, covered with earth, waiting to trap someone.
For セイ, imagine a trap so clever that you say 'Say!' in surprise when you see it: Say -> セイ, and the pitfall catches you off guard.