c. 1400, "mudhole," probably from Old English -sloppe "dung" (in plant name cusloppe, literally "cow dung"), related to slyppe "slime" (from PIE root *sleubh- "to slide, slip"). Meaning "semi-liquid food" first recorded 1650s; that of "refuse liquid of any kind, household liquid waste" (usually slops) is from 1815. Meaning "affected or sentimental material" is from 1866.
late 14c., "loose outer garment," perhaps from Old English oferslop "surplice," which seems to be related to Middle Dutch slop, Old Norse sloppr (either of which also might be the source of the Middle English word), perhaps all from Proto-Germanic *slup-, from PIE root *sleubh- "to slide, slip" on the notion of a garment one "slips" on or into (compare sleeve). Sense extended generally to "clothing, ready-made clothing" (1660s), usually in plural slops. Hence, also, slop-shop "shop where ready-made clothes are sold" (1723).