From an unused root meaning to be greasy; suet -- fat.
H6309. peder
[מֶּ֫דֶר] noun [masculine] suet, of עֹלָה (so Vrss Late Hebrew: √ unknown); — absolute מָּ֑דֶר Leviticus 1:8; 8:20; suffix מִּדְרוֺ 1:12.