Definition of blindsides:
(v) :
(transitive, figurative, informal) To catch off guard; to take by surprise.
(n) :
(figurative) A person's weak point.
(n) :
(automotive) A driver's field of blindness around an automobile; the side areas behind the driver.
(n) :
(rail transport) A tram/train driver's field of blindness around a tram (trolley/streetcar) or a train; the side areas behind the tram/train driver.
(v) :
(transitive) To attack (a person) on his or her blind side.
(n) :
(rugby union) The blindside flanker, a position in rugby union, usually number 6.