To move quickly downward to avoid hitting something, or to dodge a problem by avoiding it.
From Middle English 'douken,' possibly from Old English 'ducan' (to dive). Originally referred to the diving motion of ducks, then came to mean any quick downward movement. The avoidance meaning developed metaphorically.
The word perfectly shows how animal behavior names become verbs—we 'duck,' 'duck out,' and even 'lame duck' (a politician with no power left), all because ducks are so good at sudden diving moves!
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