@BitBun it's the origin of ß, the interior and final s together

The letter originates as the ⟨sz⟩ digraph used in late medieval and early modern German orthography, represented as a ligature of ⟨ſ⟩ (long s) and ⟨ʒ⟩ (tailed z) in blackletter typefaces, yielding ⟨ſʒ⟩.