One definition in the Oxford English Dictionary:
Pig-wash; also, wet refuse or filth.
1528 TINDALE Parable Wicked Mammon Wks. (1573) 65/1
His stomacke abhorreth longyng after slibbersause and swashe, at which a whole stomacke is readye to cast hys gorge.
1634 MERES Wits Commonw. II. 50
Swine..refuse partriges and delicats, and doe greedily hunt after Acornes and other swash.
1847 HALLIWELL, Swash..(4)
1878 Cumbld. Gloss.,
Swash, wet stuff.
1893 Times (weekly ed.) 10 Feb. 114
The well-filled troughs of swash and potatoes round which I have seen pigs crowd.