The Hunmin Jeongeum Haerye, the treatise introducing the principles behind the Korean alphabet written by its inventor King Sejong in 1446, explains that this glyph was derived from the "outline of the incisor," reflecting the sibilant nature of the consonant /s/. Compare 人, the iconic representation of teeth in the Chinese character 齒／齿(chǐ, “tooth”), originally a pictogram of a mouth full of teeth. According to Sejong, the letter ㅈ (j, “j”) was created by adding a stroke to ㅅ, because both are sibilants.
Gari Ledyard proposes that Sejong derived both ㅅ and ㅈ from the 'Phags-pa letterꡛ(s). Ledyard gives evidence that Sejong was inspired by 'Phags-pa for the basic glyph forms, although he changed the shapes of the letters drastically in order to enhance the simplicity and rationality of his script, and the ultimate shape of the letters may indeed have been influenced by that of the speech organs (Ledyard 1997).
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