means To smear is to spread a substance thinly across a surface, or, figuratively, to damage someone's reputation by spreading false or unfair accusations.

from From Old English 'smeoru,' meaning fat, grease, or ointment — the slippery stuff you rubbed onto wounds and wheels. It has Germanic cousins like German 'Schmer' (grease) and 'schmieren' (to grease or, tellingly, to bribe). The slide from literal smearing of grease to the figurative smearing of a name is a natural one: both spread something messy that's hard to wipe clean. The reputation-ruining sense is a relatively modern extension of the same greasy logic.