1. 站岗,值班：on foot 步行;在进行中 | on guard 站岗,值班 | on hand 现有;在手边;临近

2. 警惕,防范：on foot / / 步行 | on guard / / 警惕,防范 | on hand / / 在手边,临近

3. 站岗,警戒：on foot步行 | on guard站岗,警戒 | on purpose故意,有目的

adj

1. vigilant

e.g. on guard against con artists
must remain on your guard in such a situation

Synonym: on one's guard(p)upon one's guard(p)on your guard