The name derives from Terry v. Ohio, 392 U.S. 1 (1968),[2] in which the Supreme Court of the United States held that police may briefly detain a person who they reasonably suspect is involved in criminal activity;[3] the Court also held that police may do a limited search of the suspect’s outer garments for weapons if they have a reasonable and articulable suspicion that the person detained may be “armed and dangerous”.[4] When a search for weapons is authorized, the procedure is known as a “stop and frisk”.
The cop can clearly see he is armed, the person in question could be a felon and possessing a gun illegally. Do you really think a cop is going to allow someone to walk down the street possessing an open firearm and not stop to ask questions?