Old English smerian, smierwan "to anoint or rub with grease, oil, etc.," from Proto-Germanic *smerwjan "to spread grease on" (source also of Old Norse smyrja "to anoint, rub with ointment," Danish smøre, Swedish smörja, Dutch smeren, Old High German smirwen "apply salve, smear," German schmieren "to smear;" Old Norse smör "butter"), from PIE *smeru- "grease" (source also of Greek myron "unguent, balsam," Old Irish smi(u)r "marrow," Old English smeoru "fat, grease, ointment, tallow, lard, suet," Lithuanian smarsas "fat").
Figurative sense of "assault a public reputation" is by 1835; especially "dishonor or besmirch with unsubstantiated charges." Related: Smeared; smearing. Smear-word, one used regardless of its literal meaning but invested with invective, is from 1938.
"mark or stain left by smearing," 1610s, from smear (v.). Sense of "small quantity prepared for microscopic examination" is from 1903. Meaning "a quantity of cream cheese, etc., smeared on a bagel" is by 1999, from Yiddish shmir. The earliest noun sense in English is "fat, grease, ointment" (c. 1200), from Old English had smeoru "fat, grease," cognate with Middle Dutch smere, Dutch smeer, German Schmer "grease, fat" (Yiddish schmir), Danish smør, Swedish smör "butter."