The word 'swoop' is a vivid example of onomatopoeia, emerging in Middle English as 'swopen'. It is believed to imitate the sound of a swift, sweeping motion through the air, much like the related word 'sweep'. This imitative origin connects it to a family of words beginning with 'sw-' that suggest a rushing or swirling motion, such as 'swing', 'swirl', and 'swish'. Over time, its meaning evolved from the general sense of sweeping along to specifically describe the sudden, descending attack of a bird of prey, and later to any swift, decisive action of seizing or taking control.

This is a simplex word of imitative origin, likely formed to mimic the sound of rushing air or a sudden movement. Its functional role is as a complete lexical root.