BRP in chat usually refers to the Basic Role-Playing game system.
The Basic Role-Playing system is a popular generic role-playing game system created by Chaosium Inc. It is known for its simplicity and flexibility, making it easily adaptable to various settings and genres. When someone mentions BRP in a chat, they're likely discussing or referring to games that use or are based on this system. Examples of games that use the BRP system are Call of Cthulhu, RuneQuest, and Stormbringer.
