blindside

Short definition from Wiktionary

Noun

1. 1
   A driver's field of blindness around an automobile; the side areas behind the driver.
2. 2
   A tram/train driver's field of blindness around a tram (trolley/streetcar) or a train; the side areas behind the tram/train driver.
3. 3
   A person's weak point.
4. 4
   The space on the side of the pitch with the shorter distance between the breakdown/set piece and the touchline; compare openside.
5. 5
   The blindside flanker, a position in rugby union, usually number 6.

Verb

1. 1
   To attack (a person) on his or her blind side.
2. 2
   To catch off guard; to take by surprise.