Like most sticky internet slang terms, "slop" stems from 4chan's "sloppa", initially used to describe gross looking food, i.e. "slop of shit" shortened to "slop

Like most sticky internet slang terms, "slop" stems from 4chan's "sloppa", initially used to describe gross looking food, i.e. "slop of shit" shortened to "sloppa" Now used to describe anything that looks half-assed, poorly put together, etc.