keep an eye out for
Also, keep a sharp lookout for. Be watchful for something or someone, as in Keep an eye out for the potholes in the road, or They told him to keep a sharp lookout for the police. The first expression, sometimes amplified to keep a sharp eye out for, dates from the late 1800s, the variant from the mid-1700s. Also see have one's eye on, def. 1; keep a weather eye; keep one's eyes open; look out.
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