1
general::
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 mid1700s. Also see HAVE ONE'S EYE ON, def. 1; KEEP A WEATHER EYE; KEEP ONE'S EYES OPEN; LOOK OUT.
American Heritage Idioms