A slop bowl is one of the components of the traditional tea set

Tea set

A tea set, in the Western tradition, is a set of dishes sold in a group for use at afternoon tea or a formal tea party.Tea sets vary greatly in quality and price, from inexpensive to high-end.-Items in a tea set:...

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In modern times, the slop bowl is used to hold the discarded hot water that was used to warm the teapot

Teapot

A teapot is a vessel used for steeping tea leaves or a herbal mix in near-boiling water. Tea may be either in a tea bag or loose, in which case a tea strainer will be needed, either to hold the leaves as they steep or to catch the leaves inside the teapot when the tea is poured...

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Earlier uses may have included emptying undrunk cold tea

Tea

Tea is an aromatic beverage prepared by adding cured leaves of the Camellia sinensis plant to hot water. The term also refers to the plant itself. After water, tea is the most widely consumed beverage in the world...

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