Always?

https://en.wikipedia.org/wiki/Slop_(clothing)

In 16th- to 19th-century Europe and North America, the slop trade was the manufacture and sale of slop, cheap ready-made clothing

By semantic change, "slop", which had once related to the procurement of naval clothing, was applied generally to ready-made garments. By a further semantic change during the 19th century, "slop" became a term applied to all clothing that was not skillfully tailor-made for the customer: so might refer to machine-cut and machine-stitched garments, those made by apprentices or those less skilled, and those sold second-hand. The connotation of low quality carried over to boots, and furniture.

Words do change over time.