Planchar (Spanish for “to iron”) comes from the French for the same, planche, which comes from the Latin plancus, for “straight.” Ironing is making something straight!

From that same root, we get the English… plank. A plank, after all, is just a piece of wood that is… straight. 

The mapping of the Spanish p‑l-n-ch to the English p‑l-n‑k is quite clear.