From Tapestries MUCK
MUF is a programming language based on Forth. It is used as an application extension language for TinyMUCKs, and is sometimes expanded to mean Muti-User Forth, though it is unknown if this is a backronym or not.
The ability to program in MUF is restricted on Tapestries due to it's security problems. At this point there are only a very few people left who have permission to write MUF programs, and no new people are being accepted outside of the Wizards.