pango:font-description-weight (desc)

desc -- a pango:font-description instance
weight -- a pango:weight value or an integer for the weight of the font description

Gets or sets the weight field of a font description. The weight field specifies how bold or light the font should be. In addition to the values of the pango:weight enumeration, other intermediate numeric values are possible.

(setq desc (pango:font-description-new))
=> #<PANGO:FONT-DESCRIPTION {1005A11E63}>
(pango:font-description-weight desc)
=> :NORMAL
(setf (pango:font-description-weight desc) 450)
=> 450
(pango:font-description-weight desc)
=> 450