Final answer:
The term Whirlybird is an informal term for helicopters, especially those from military contexts. It is assumed to have originated in the early 1950s and is descriptive of the sound and the whirling action of helicopter blades.
Step-by-step explanation:
The term Whirlybird is believed to be an vernacular or slang term for helicopters, in particular those used in military contexts. Its origin can be linked back to the early 1950s when helicopters began to be commonly used in warfare. The term is descriptive, as it mimics the sound and the action (whirling or rotating) of the helicopter blades. Thus, the term 'whirlybird' reflects both the sound a helicopter makes and the rotatory movement of its rotor blades.
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