What is the meaning of the word "guard"?

Definitions:

(American football) a position on the line of scrimmage noun
- guards must be good blockers
a posture of defence in boxing or fencing noun
- keep your guard up
take precautions in order to avoid some unwanted consequence verb
- guard against becoming too friendly with the staff
the duty of serving as a sentry noun
- They were on guard that night.
the person who plays that position on a football team noun
- the left guard was injured on the play
to keep watch over verb
- there would be men guarding the horses