四萬咁口 - A person smiling very happily. It literally means "the mouth is like the four wan tile". (Some people suggest that the origin is that the person really smiles so happily that his mouth does look like the Chinese character for 'four'.)
叫和 - Something is about to finish or happen with a favorable result. It is used, for example, to describe the favorable player or team at match point. Literally, it means the player has a "ready hand". He has the chance to win the round once a favorable tile comes up.
食詐和 - A person thinks he has had a lucky encounter, but then it turns out it was only a mistake. The literal meaning in a Mahjong game is that the player mistakenly believes that he has won the round and announces this, only to find he made a mistake and thus becomes a loser.
俾隻好牌你上 - literally "giving you a good tile to chow" (that is, "giving you a good tile to use to piece together a winning combination"), it means somebody setting up another person up for something positive. For example, setting one's friend for an ideal date or job.
食雞和 - A player has won the game, but with a hand that has no sets of three from the same suit, or having no combinations which give points etc.
小相公 - Any case when a player's total number of tiles are fewer than the required.