late 14c., "generously, munificently," from liberal (adj.) + -ly (2). Meaning "freely" is c. 1500.

1540s, "conveniently," from handsome + -ly (2). Meaning "attractively" is from 1610s; "liberally, generously" from 1735.

c. 1200, "liberally, generously, bountifully;" also "in large measure; abundantly," from large + -ly (2). Meaning "extensively, to a great extent" is c. 1400.

"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.