The Spanish espalda (“back”, meaning the part of your body where your spine is!) comes from the Latin spatula meaning also “back” (same sense) or “short wooden blade.”
It’s hard to predict what English word came from the same root? Spatula! It’s easy to see how this evolved: a short wooden blade can look like your shoulder. (Hence the French word for “shoulder” is épaule!)
The s-p-t of spatula maps clearly to the s-p-(l)-d of espalda.