guard has 10 distinct synonyms spread across 5 of its 14 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a person who keeps watch over something or someone
the person who plays that position on a football team
“the left guard was injured on the play”
a device designed to prevent injury or accidents
a posture of defence in boxing or fencing
“keep your guard up”
the person who plays the position of guard on a basketball team
a military unit serving to protect some place or person
a precautionary measure warding off impending danger or damage or injury etc.
“he put an ice pack on the injury as a precaution”
the duty of serving as a sentry
“he was on guard that night”
(American football) a position on the line of scrimmage
“guards must be good blockers”
to keep watch over
“there would be men guarding the horses”
take precautions in order to avoid some unwanted consequence
“guard against becoming too friendly with the staff”
Synonyms, antonyms and the broader and narrower relations for guard come from Princeton University’s WordNet. Full sources and licences.