From The Collaborative International Dictionary of English v.0.48 [gcide]:
Smooth-spoken \Smooth"-spo`ken\, a.
Speaking smoothly; plausible; flattering; smooth-tongued.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
smooth-spoken
adj 1: expressing yourself readily, clearly, effectively; "able
to dazzle with his facile tongue"; "silver speech" [syn:
eloquent, facile, fluent, silver, {silver-
tongued}, smooth-spoken]