Sludge is a substance that is between solid and liquid form.(noun)
See sludge in Webster's New World College Dictionary
Origin: var. of slutch, sludge, mud: akin to ME sluchched, muddy, prob. ult. < IE base *(s)leu-, to hang limply > sleet, MHG slote, mud, ooze
See sludge in American Heritage Dictionary 4
Origin: Perhaps alteration of dialectal slutch, mire.
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