noun: (biology) A round lump made up of or coated with slime or a slime-like substance such as mucus.
noun: (helminthology, specifically) A mucus-coated lump containing the cercariae (“parasitic larvae”) of a liver fluke (of the phylum Platyhelminthes).
noun: (originally US, colloquial, derogatory) A person who is regarded as slimy (that is, sneaky or underhanded) or otherwise undesirable.