The unwritten, unspoken rules of polite society. It's what keeps you from laughing or commenting on bodily functions that happen in public and ignoring homeless/smelly people until you can escape their visual/audible range.
May also apply to catching someone doing something inappropriate in public, in which both parties act like nothing happened and/or nothing was seen.
I went into the bathroom to wash my hands just as David was flushing the toilet. His shit stunk so bad, it was all I could think about. I tried to hold my breath and ignore it as the social contract keeps me from acknowledging the fact that his shit smell was suffocating me.