A runner-runner refers to improving a hand on both the turn and river to make the winning hand. Although a rare occurrence, when it does happen, they tend to be runner-runner flushes or straights. It is a bad beat in the truest sense of the word and when a player catches the perfect runner-runners it can make the losing player go on massive tilt.
EXAMPLE “I had my opponent crushed with a flopped straight but he made a runner runner boat to win a decent sized pot.”
“In an EPT super high roller event hand, Olivier Busquet came from 0.4% to win in runner-runner fashion.”