What is the meaning of Blindside?

A driver's field of blindness around an automobile; the side areas behind the driver.

A tram/train driver's field of blindness around a tram (trolley/streetcar) or a train; the side areas behind the tram/train driver.

A person's weak point.

The space on the side of the pitch with the shorter distance between the breakdown/set piece and the touchline; compare openside.

The blindside flanker, a position in rugby union, usually number 6.

To attack (a person) on his or her blind side.

To catch off guard; to take by surprise.

Source: wiktionary.org