From AutoCAD help:
LWDEFAULT
Type: Enum
Saved in: Registry
Initial value: 25
Sets the value for the default lineweight. The default lineweight can be set to any valid lineweight value in hundredths of millimeters, including: 0, 5, 9, 13, 15, 18, 20, 25, 30, 35, 40, 50, 53, 60, 70, 80, 90, 100...