Glib
Etymology
Related to older Germanic words for "slippery," such as Low German glibberig, "slick, slimy"; earlier English even used "glib" to mean literally smooth or slippery. The metaphor is the whole meaning: glib speech slides out too easily and too smoothly to be trusted. Fluency without depth or sincerity is the enduring sense.
adjective ยท /GLIB/
Fluent and persuasive in a smooth but shallow, insincere way.
What it names
The too-easy eloquence of someone who always has a slick answer ready and never seems to have thought hard about it.
Example
He gave a glib excuse about traffic that neither of them believed for a second.
Related idea
Eloquent is fluent speech that carries real substance; glib is fluency that has traded substance for smoothness.