go south

Short definition from Wiktionary

Verb

1. 1
   To become unfavorable; to decrease; to take a turn for the worse.
2. 2
   To make an escape; to disappear.
3. 3
   To give somebody oral sex.
4. 4
   To rathole: to take chips off the table in a cash game without exiting the game completely, especially with the intention of limiting losses.