The term white trash first came into common use in the 1830s as a pejorative used by house slaves against poor whites. In 1833 Fanny Kemble, an English actress visiting Georgia, noted in her journal: “The slaves themselves entertain the very highest contempt for white servants, whom they designate as ‘poor white trash’”
...By 1855 the term had passed into common usage by upper class whites, and was common usage among all Southerners, regardless of race, throughout the rest of the 19th century.
Don't kid yourself; Black slaves didn't invent the term. Slave-owners encouraged slaves to look down on poor whites so they would be more satisfied with their lot. They did the same with the poor whites, teaching them to hate Blacks and hiring them to serve as their "patter rollers" (slave patrols). It began in the seventeenth century as a "divide and rule" strategy and it's still working.