Under http://en.wikipedia.org/wiki/MUD I found this:

"In 1978 Roy Trubshaw, a student at Essex University in the UK, started working on a multi-user adventure game in the MACRO-10 assembly language for a DEC PDP-10. He named the game MUD (Multi-User Dungeon), in tribute to the Dungeon variant of Zork, which Trubshaw had greatly enjoyed playing.[18] Trubshaw converted MUD to BCPL (the predecessor of C), before handing over development to Richard Bartle, a fellow student at Essex University, in 1980.[19][20][21]

MUD, better known as Essex MUD and MUD1 in later years, ran on the Essex University network until late 1987,[22] becoming the first Internet multiplayer online role-playing game in 1980, when Essex University connected its internal network to ARPANet."