Mezcla (Spanish for “mix”) comes from the Latin miscere, meaning, “to mix.” You can envision the sound change when you remember that the ‑sc- sound sounds and even looks like the letter ‑zc-!

From the same Latin root miscere we get the English, promiscuous — just miscere with the emphasis prefix pro-, so it literally means “to mix indiscriminately.” What does a promiscuous girl (or, ummm, guy) do if not mix with anyone without discriminating between them that much?

The m‑z-c of mezcla clearly maps to the m‑s-c of promiscuous.