You can only undefine built-in AutoCAD commands. You cannot undefine the following
types of commands:

• Commands defined by AutoLISP ^آ® . This includes ObjectARXâ„¢ application commands registered by acedDefun().
• External commands and aliases defined in the acad.pgp file.

If an AutoLISP or ObjectARX application has redefined a command with the same name
as a built-in AutoCAD command, the application-defined command is active.

You can restore an undefined command with REDEFINE.

Use the following syntax to run undefined commands or registered ObjectsARX commands:

• To run an undefined built-in AutoCAD command, precede the command name with a period
  (.).
• To run ObjectARX application commands that are registered by acedRegCmd, precede the command name with a period (.), the command’s group name, and another
  period (.). For example, the MTEXT command can be accessed with .acad_mtext.mtext.To see a listing of command names and groups of an ObjectARX application, use the
  ARX command, and choose the Commands option.

The following prompt is displayed.

Command name

Enter the name of the command you want to suppress.

This undefined command can be redefined to perform another function.