匸 is the right-opening box radical, drawn as two strokes wrapping something on the left. It depicts a hiding-place or screened nook — exactly what something tucked inside 匿 is meant to be sheltered behind.
若 sits inside the wrapper and supplies a phonetic that has drifted from ruò to nì, with both readings sharing nasal-final endings in older Chinese. The whole character pictures a person pressed into a hiding-screen, hence the meaning to conceal.
