Shake the pan occasionally to distribute the sauce and slosh it over the fish.
1814, "slush, sludge, a watery mess," probably a blend of slush and slop (n.1) in its Middle English sense of "muddy place."
"to splash about in mud or wet," 1844, from slosh (n.). Meaning "to pour carelessly" is recorded from 1875. Related: Sloshed; sloshing.