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):
from Travels in Alaska by John Muir (describing a trip in 1879):