high mountain, precipitous
磪 centers on towering, steep terrain: a high mountain or a precipitous cliff face. Both given meanings describe the same imposing vertical landscape.
磪 combines 石 (stone) with 崔, which itself contains 山 (mountain) and 隹 (bird). The character likely expresses a high, rocky mountain through its stone and mountain components.
Stone 石 on the left and 崔 (which contains mountain 山) on the right: picture a massive stone mountain rising steeply, its peak lost in clouds.
For サイ, imagine a sigh escaping your lips as you gaze up at the precipitous peak: sigh -> サイ, the sound of awe at the high mountain.