"spread liberally," 1847, of uncertain origin. Early 19c. local glossaries from western England have the word with a sense "to slip or slide."
Slather on the manure on all the hoed crops, if you have it; if not buy of your improvident neighbor. [Genesee Farmer, June 1847]
Sometimes said to be from a dialectal noun meaning "large amount" (usually as plural, slathers), but this is first attested 1855. Related: Slathered; slathering.