L&M define this as "seamen's ready made clothes".
1. slops Articles of clothing and bedding issued or sold to sailors.
2. slops Short full trousers worn in the 16th century.
3. A loose outer garment, such as a smock or overalls.
4. slops Chiefly British. Cheap, ready-made garments.
[Middle English sloppe, a kind of garment, from Old English -slop (in oferslop, surplice).]