What is origin of word airfoil (e.g. wings of the airplane)? Why it has part "foil", which is a very thin metal sheet.
"Dictionary.com Unabridged" says only "Origin: 1920–25; air 1 + foil 2" (© Random House), and "foil 2" is defined by them exactly as thin metal, e.g. aluminium foil (http://en.wikipedia.org/wiki/Foil_(metal)).
Best Answer
It comes from the notion of a foil as a leaf shaped curve formed by the cusping of an arch or circle. (See also the definition of a foil as a blade for fencing.)
An Airfoil is a similarly leaf-shaped curve or blade that is used to push/cut the air. Look at the cross sections of common airfoil shapes, and you will see the similarity to the shape of a leaf.
Foil is derived from the latin folium meaning leaf.
Incidentally (and not directly related to the rest of the answer), that is the same etymology of the term gold leaf to mean a thin gold foil (leaf = foil).