The modern form of し comes from the cursive form of the kanji 之 (し, shi, i.e. “of”, “this”). Follow the progression of the different scripts shown in Figures 1 to 3 to understand its origin.
Figure 1. Standard script (楷書, かいしょ, kaisho) of the kanji 之.
Figure 2. Semi-cursive script (行書, ぎょうしょ, gyōsho) of the kanji 之.
Figure 3. Cursive script (草書, そうしょ, sōsho) of the kanji 之.