"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.
1812 (n.) "a follower of English economist Thomas R. Malthus (1766-1835)," especially with regard to the doctrines set forth in his "Essay on the Principle of Population" (published anonymously in 1798).
In this work he first made prominent the fact that population, unless hindered by positive checks, as wars, famines, etc., or by preventive checks, as social customs that prevent early marriage, tends to increase at a higher rate than the means of subsistence can, under the most favorable circumstances, be made to increase. As a remedy he advocated the principle that society should aim to diminish the sum of vice and misery, and check the growth of population, by the discouragement of early and improvident marriages, and by the practice of moral self-restraint. [Century Dictionary]
As an adjective, "of or pertaining to Malthus," by 1818. Related: Malthusianism "theory of the relation of population to the means of subsistence" (1825). The surname is attested from late 13c., and probably means "worker at a malt-house."