Flank

Flanks are the exposed ends of a formation of fighters. They are the achilles' heel in any battlefield situation, for the simple reason that most people can't fight in two directions at once.

Flanking is the practice of getting around these exposed ends and attacking your enemy from the side or back. Being flanked puts the victim at a terrible disadvantage, as it completely disrupts the defense and often causes confusion and panic.

A flank can also refer to the blind spots just behind an individual's shoulders and back; which has been referenced in any number of bawdy lyrics.