A slop bowl is one of the components of the traditional tea set.
In modern times, the slop bowl is used to hold the discarded hot water that was used to warm the teapot.
Earlier uses may have included emptying undrunk cold tea into the slop bowl before refilling the cup with fresh, hot tea. Often the slop bowl was used to empty the dregs of the tea cup before refilling with hot tea, because there were often tea leaves left behind in the bottom of the cup.
References
Slop bowl Wikipedia(Text) CC BY-SA