guard dog

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Also see: dog

Concise Oxford English Dictionary © 2008 Oxford University Press:
guard/ɡɑːd/

▶verb

• 1 watch over in order to protect, control, or restrict.

  ■ Basketball stay close to (an opponent) to prevent them getting or passing the ball.

• 2 prevent from being harmed.

  ■ (guard against) take precautions against.

▶noun

• 1 a person, especially a soldier, who guards or keeps watch.

  ■ N. Amer. a prison warder.

• 2 [treated as sing. or pl.] a body of soldiers serving to protect a place or person.

  ■ (Guards) the household troops of the British army.

• 3 a device worn or fitted to prevent injury or damage.

• 4 a defensive posture adopted in a fight.

  ■ vigilance against adverse circumstances: he let his guard slip.

• 5 Brit. an official who rides on and is in general charge of a train.

• 6 Basketball each of two players chiefly responsible for marking opposing players.

• 7 American Football each of two players either side of the centre.

– phrases
guard of honour a group of soldiers ceremonially welcoming an important visitor.
off (or on) guard unprepared (or prepared) for a surprise or difficulty.
take guard Cricket (of a batsman) stand in position ready to receive the ball.

– origin ME (in the sense ‘care, custody’): from OFr. garde (n.), garder (v.), of W. Gmc origin; cf. ward.