From Longman Dictionary of Contemporary Englishon your guardon your guardCAREFULto be paying attention to what is happening in order to avoid danger, being tricked etc These men are dangerous so you’ll need to be on your guard. Something in his tone put her on her guard. → guard
Examples from the Corpus
put ... on ... guard• The hostile bark put Miguel on his guard.• Anyway after that he put Raimundo on guard with a gun.• Enright at the Freie Universität in West Berlin, and that information immediately put me on my guard.• Whatever, it puts you on your guard.• The poll tax disaster should surely put us on our guard.• And officers - some armed - were put on guard at Aylesbury police station.• Detectives feared he was out for revenge and armed police were put on guard at police stations. 