1
general::
Phrase(s): keep (close) watch (on someone or something)
to monitor someone or something; to observe someone or something. • Keep close watch on Bill. I think he’s loafing. • Okay. I’ll keep watch, but I think he’s a good worker., Phrase(s): keep (close) watch (over someone or something)
to guard or care for someone or something. • I’m keeping watch over my children to make sure they have the things they need. • I think that an angel is keeping close watch over her to make sure nothing bad happens to her.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Also, keep a watch or close watch on;watch over. Observe with continuous attention, especially to act as a sentinel or for protection. For example, Afraid that the wolves would return, she kept watch while the others slept, or They kept a close watch on the harbor, looking for signs of enemy ships, or, according to the Gospel of St. Luke (2:8): "And there were in the same country shepherds . . . keeping watch over their flock by night." [Late 1300s] Also see KEEP AN EYE OUT FOR.
American Heritage Idioms