It is said, and we believe it to be true, that there are far more English words having insulting or derogatory meaning than there are those carrying a complimentary connotation.
Certainly slubberdegullion would rank among the most degrading of epithets short of foul speech.
For slubber is an older form of slobber, “to befoul as with saliva, to slaver.”
The de may be a meaningless connective, or may be in imitation of the French de, “of.”
Gullion seems to be a variant of cullion, “a vile or despicable fellow,” from the French couillon (colon), “a dastard, coward.”
Hence both the etymological and actual meanings of slubberdegullion: “a slobbering, worthless sloven.”