"quality of giving or bestowing liberally or lavishly," early 15c., from Old French munificence, from Latin munificentia "bountifulness, liberality, generosity," from stem of munificus "generous, bountiful, liberal," literally "present-making," from munus "gift or service; function, task, duty, office" (see municipal) + unstressed stem of facere "to do" (from PIE root *dhe- "to set, put").
"spread liberally," 1847, a word of uncertain origin. Early 19c. local glossaries from western England have the word with a sense "to slip or slide" (1809).
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), of obscure origin, but this is first attested 1855. Related: Slathered; slathering.