windmill

Concise Oxford English Dictionary © 2008 Oxford University Press:
windmill/ˈwɪndmɪl/

▶noun

• 1 a building with sails or vanes that turn in the wind and generate power to grind corn into flour.

  ■ a similar structure used to generate electricity or draw water.

• 2 Brit. a toy consisting of a stick with curved vanes attached that turn in the wind.

• 3 a propeller, especially one on an autogiro.

▶verb

• 1 move (one's arms) in a manner suggestive of the sails of a windmill.

• 2 (of a propeller, rotor, or aircraft) spin unpowered.

– phrases
fling (or throw) one's cap over the windmill (s) dated act recklessly or unconventionally.