Note: This word has traditionally been traced back to Middle English sloppes, attested once, in the alliterative Morte Arthure (ca. 1440), and then further traced to Old English -sloppe in cusloppe, a variant of cūslyppe cowslip, in which -sloppe is taken to mean "dung, excrement." The passage in the Morte Arthure in which sloppes occurs describes Arthur and his knights disembarking from boats and wading ashore: "Londis als a lyon with lordliche knyghtes, / Slippes in the sloppes o slante to þe girdyll" (lines 3922-23). The Middle English Dictionary tentatively defines the word as "muddy waters." Note that it forms an alliterative pair with slippes, from which it differs only by a vowel. No further attestations of slop appear before the seventeenth century, and then again often paired with slip. The sense "medicine in the form of a tasteless drink or liquid food" occurs in 1658 as slops, in 1668 as slip-slops. Though slip-slop has been taken as a compound based on slop, the possibility that the compound is actually based on slip suggests itself—in which case slop would be an affective coinage that may have been made more than once and is not datable to Old English. Compare slop entry 3.

Note: The sense "to lap up," attested since the sixteenth century, may be of independent origin. The Oxford English Dictionary's citation from Thomas Tusser's A Hundreth Good Pointes of Husbandrie, printed in 1557 ("Their milke slapt in corners, their creame al to sost"), placed under the sense "to spill or splash (liquid)," is 250 years earlier than the next citation and probably an instance of a different word (slab?).