Left mound radical, the left-side form of
阜, picturing a stepped hillside or steep slope. It supplies the meaning: dangerous terrain, treacherous heights.
险 originally meant a steep mountain pass — a place of physical peril — and from there generalised to 'danger, risk, hazard' in any sense. Same family includes
阵 (formation),
防 (defend),
阻 (block),
隘 (narrow pass).
