On a plane, the engine often sits inside a smooth metal shell. That shell is the cowling. It keeps rain, dirt, and stones off the engine, and it helps the air slide past so the plane can fly cleanly. Mechanics take the cowling off when they need to look at the engine, then put it back on. You will hear this word around aircraft and in flying talk; in ordinary speech people may just say “the cover on the engine.” It is not a posh word, but it is a specialist one — if you never go near planes, you may never need it. Though actually, a cowl was first a deep hood, like the hood a monk wears, and the plane part is the same idea: a hood for the engine.