The meaning of the sack
Bunuel was an avid reader of the early Church Fathers (see The Milky Way, for instance) and without a doubt knew of the comment expressed by several divines, notably Bishop Odo of Cluny, a 9th century French cleric, that to embrace a woman was to embrace a sack of manure. The valet even says this in the film, attributing it to 'a friend'.
Fernando Rey spends much of the movie toting a bag of excrement around or leaving it with others, clearly representing his mistress, the absolutely infuriating Conchita.