Sweet-talk, 1935, from noun phrase; see sweet (adj.) + talk (n.). Earliest usages seem to refer to conversation between black and white in segregated U.S.
"I ain' gonna stay heah no longah. Don' nevah keer, ef I do git cotched--or die. Tha's bettah than to stay heah an' listen to Maw Haney sweet-talk the white folks, whilst they drives us clean to the grave. ..." ["The Crisis," July 1935]
Latin had suaviloquens, literally "sweet-spoken."