A variant or archaic spelling of 'cowl,' a hood or hooded garment, or the removable cover of an aircraft engine.
From Latin 'cuculla' (a hood or cowl), which traveled through Old English and Middle English. The spelling 'cowle' represents an older orthographic variation before Modern English standardized spelling in the 17th and 18th centuries.
Medieval monks wore cowls so distinctively that their robes became defined by the hood itself—which is why we now use 'cowl' for any protective hood, including on airplane engines where it serves a completely different but parallel function!
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