Question: What does “PIE” mean?

Answer: A “PIE” is a limit set by players. Players start with $5 in coins to pay the winner. When a player loses their “PIE”, they continue to play but they are exempt from paying until they win money back. Some players say that “PIE stands for “purse is empty”. The league calls it a “PY” but we don’t know why. This ruling is not found in Mah Jongg Made Easy.

(See also https://mahjcon.com/mahjpedia/id53)

Source:  National Mah Jongg League Bulletin (1967)