glib

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Performed with a natural, offhand ease.
• adjective Given to or characterized by fluency of speech or writing that often suggests insincerity, superficiality, or a lack of concern.

from The Century Dictionary.

• Smooth; slippery: as, ice is glib.
• Running smoothly or sleekly; plausibly voluble: as, a glib tongue.
• To run smoothly; move freely, as the tongue.
• To make smooth; cause to run smoothly, as the tongue; make glib.
• noun A bushy head of hair, formerly common among the Irish. See the extracts.
• noun A man wearing such a bush of hair.
• To castrate.

from the GNU version of the Collaborative International Dictionary of English.

• transitive verb obsolete To make glib.
• noun obsolete A thick lock of hair, hanging over the eyes.
• adjective obsolete Smooth; slippery.
• adjective Speaking or spoken smoothly and with flippant rapidity; fluent; voluble
• transitive verb obsolete To castrate; to geld; to emasculate.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb obsolete To castrate; to geld; to emasculate.
• adjective Having a ready flow of words but lacking thought or understanding; superficial; shallow.
• adjective Smooth or slippery.
• adjective Artfully persuasive in nature.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective having only superficial plausibility
• adjective artfully persuasive in speech
• adjective marked by lack of intellectual depth