From 御(お) (o-, honorific prefix) + 負(ま)け (make, 連(れん)用(よう)形(けい) (ren'yōkei, “stem or continuative form”) of the verb 負(ま)ける (makeru, “to lower in price”)).
おまけ or オマケ • (omake) ←おまけ (omake)?