By convention, the bottom side of a map is south.
True south is the direction towards the southern end of the axis about which the earth rotates, called the South Pole. The South Pole is located in Antarctica. Magnetic south is the direction towards the south magnetic pole, some distance away from the south geographic pole.
The word south is derived from the name of one of the four dwarves in Norse mythology, Norðri, Suðri, Austri and Vestri, who each represented one of the directions of the world. The etymology of South can be traced back to the Old English word suth, related to the Old High German word sund, and perhaps sunne in Old English with sense of "the region of the sun."