From Longman Dictionary of Contemporary Englishslatherslath‧er /ˈslæðə $ -ər/ verb [transitive always + adverb/preposition] American English COVERto cover something with a thick layer of a soft substance SYN smotherslather something in/with/on something a slice of homemade bread, slathered with jam→ See Verb table
Examples from the Corpus
slather• He was already slathering Miracle Whip on that brand of soft white bread that can double as a foam sponge.• toast slathered with butter• Dearest Ethel, I can imagine myself kissing your pretzel-like body and slathering you with various oils and oatmeal. 