Lord of the crossing
From A Wiki of Ice and Fire
Lord of the crossing is a game played at the Twins. It usually involves lots of hitting and arguing. The purpose of the game is to be the lord of the crossing, usually a player standing on a bridge over water.
The game must be played on some sort of bridge placed over water of some sort. One player, the "lord of the crossing," stands in the middle of the bridge with a staff. When another player approaches, the lord of the crossing must say "I am the lord of the crossing, who goes there?" The player who approached must then present his reasons for crossing the bridge and why he should be allowed to cross. The lord asks the player questions and makes them swear oaths. The player does not have to respond truthfully to the questions but the oaths are binding unless the player says "Mayhaps" quickly enough that the lord does not notice. Then the player must attempt to knock the lord off the bridge. The lord can knock a player into the water at any time, and he is the only one armed with a staff. Only when the lord is displaced can another player become lord, but only if they said mayhaps in the game, otherwise it means immediate disqualification.
In the Novels
Before the Red Wedding, the embittered Lord Walder Frey speaks to Robb Stark, saying "You wanted crossing and I gave it to you, and you never said mayhaps, heh." This hints to the reader, but not to Robb, at some sort of approaching turnabout.