c.1400, "mudhole," probably from Old English -sloppe "dung" (in plant name cusloppe, literally "cow dung"), related to slyppe "slime" (see slip (v.)). Meaning "semiliquid 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," probably from Middle Dutch slop, of uncertain origin, corresponding to words in Old Norse and perhaps in Old English. 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).
"to spill carelessly" (transitive), 1550s, from slop (n.1). Intransitive sense from 1746. Related: Slopped; slopping.