“The tasteless meaning of ‘tacky’ originated in the American South, where the word originally referred to a scrawny or broken-down horse….
“Within a few decades, ‘tacky’ had extended to humans, serving as a self-deprecating label for poor white Southerners who were identified with their equine counterparts. As a North Carolinian wrote in an 1836 letter documented in Norman E. Eliason’s book ‘Tarheel Talk,’ ‘I tell them I don’t know any better for I’m a mountain tackey sartin [certainly].’
“The word then made the move from noun to adjective. A writer from Charleston, S.C., explained in 1890 that ‘tacky’ applied to ‘persons of low ideas and vulgar manners, whether rich or poor’ who exhibit ‘an absence of style.’ Clothing, he said, was considered tacky if it was ‘cheap and yet pretentious.’
“But that gaudy style wasn’t always a source of shame. Also in 1890, a Kentucky correspondent for the journal Dialect Notes reported that ‘recently we have had “tacky parties,” where the guests dress in the commonest and most unfashionable costumes.’ Such parties (often featuring awards for tackiest costumes) persisted throughout the South, particularly in Alabama, Georgia and the Carolinas, according to the Dictionary of American Regional English.”
— From “The Gauche Origins of the Word ‘Tacky’ “ by Ben Zimmer in the Wall Street Journal (July 18, 2014)