Sets the thickness of the line entity (DXF 39) in drawing units. Thickness is the extrusion length along the normal. A positive value extrudes along the normal. A negative value extrudes opposite from the normal. A zero value defines a line without thickness. The initial value is zero by default. 

Example of Working with the Line Object 

Specific Line Properties 

OdDbLine::thickness() method

OdDbLine Methods