In earlier use (late 1800s), it meant a cheap or unpleasant beverage.
Before that, it was a term for the muddy waste in a mining sluice; this meaning originated with California gold miners in the mid-1800s.
So while the exact definition might change over time, the basic concept of an unpleasant semi-liquid remained constant until recently. One can now find proper recipes for slumgullion stew, some of which sound quite good.
Etymology: Possibly Old English slum, slime + gullion, mud or cesspool, to derive the mining term.
from Roughing It by Mark Twain (1872):
Then he poured for us a beverage which he called "Slum gullion", and it is hard to think he was not inspired when he named it. It really pretended to be tea, but there was too much dish-rag, and sand, and old bacon-rind in it to deceive the intelligent traveler.
from Travels in Alaska by John Muir (describing a trip in 1879):
The meals are all alike - a potato, a slice of something like bacon, some gray stuff called bread, and a cup of muddy, semi-liquid coffee like that which the California miners call "slickens" or "slumgullion".
