on guard

transitive verb

1. to keep safe from harm; watch over and protect; defend; shield
2. to watch over; specif.,
   1. to keep from escape or trouble
   2. to hold in check; control; restrain
   3. Sports to keep (an opponent) from making a gain or scoring; also, to cover (a goal or area) in defensive play
   4. to supervise entrances and exits through (a door, gate, etc.)
3. to cover (a piece of machinery) with a device to protect the operator
4. Archaic to escort

intransitive verb

1. to keep watch; take precautions (against)
2. to act as a guard

noun

1. the act or duty of guarding; careful watch; wariness; defense; protection
2. 1. a posture of alert readiness for defense, as in boxing, fencing, etc.
   2. the arms or weapon in such a posture
3. any device that protects against injury or loss; specif.,
   1. the part of the handle of a sword, knife, or fork that protects the hand
   2. a chain or cord attached to a watch, bracelet, etc. to protect against loss
   3. a ring worn to keep a more valuable ring from slipping off the finger
   4. a safety device, as in machinery
   5. an article worn to protect a part of the body, as in a sport: a catcher's shin guards
4. a person or group that guards; specif.,
   1. a sentinel or sentry
   2. a railway brakeman or gatekeeper
   3. Brit. a railroad conductor
   4. a person who guards prisoners
   5. a special unit of troops assigned to the British royal household
   6. a military unit with a special ceremonial function: a color guard
5. ☆ Basketball either of two players who are the main ball handlers and offensive leaders positioned at the rear of the court on offense
   see point guard
6. ☆ Football either of two players on offense at the left and the right of the center whose purpose is blocking
   see also nose guard