A muddy pond or swampy area filled with soft mud and decaying organic matter.

(idiom) like trying to plaster a wall with mud; describes someone who is inept, lacks ambition, or is impossible to help or improve despite support

(proverb) literally meaning mushy mud cannot be used to plaster a wall; used to describe someone who is so inept or worthless that they cannot be helped or improved no matter how much support they receive