It's an animal but also something you shout. Find out when you use it:

BBC Learning English

Duck!

 

Thousands of plastic ducks float down the River Thames in the Great British Duck Race. Photo: Anthony Devlin/ PA Wire

Today's Phrase

If you tell someone to duck it means that they should lower their head because something is about to hit them.

Examples:
As I walked in the door someone shouted "Duck!" But it was too late and I hit my head.

The batsman shouted "Duck!" as he hit the ball right at where the spectators were standing.

Take note

To duck out means to try and escape or get out of doing something.

Example:

Sorry, I've got to duck out of this meeting, I've got too many other things to do.

Interesting fact

The yellow duck in the photograph was among around 250,000 other blue plastic ducks floating along a one kilometre stretch of the River Thames near Hampton Court Palace in London. This Great British Duck Race raises money for charity.

Source:  http://www.bbc.co.uk/worldservice/learningenglish/language/2013/06/130611_todays_phrase_duck.shtml