1. noun A response from MUD to a ridiculous command which nonetheless somehow ought to do something. Examples: 'play poker' using the kind of poker meant for poking fires; 'put pin in effigy' using a rolling pin.
2. noun A response from MUD to a combination of normally disparate words which happen to make some kind of sense if used together. Example: 'fish finger'.
3. noun A response from MUD to a command which would require unreasonable depth or breadth to deal with or is downright impossible, eg. 'get air'.

All these variations are related, but the first kind of silly is regarded as the most rewarding. Some real addict explorers spend the bulk of their playing time looking for sillies.