The adjective glib (describing insincere or shallow language) was first attested in 1594, when it meant "slippery" or physically inconsistent. About a decade later, this was figuratively applied to denote speech with those qualities. The word is a shortening of the (now antiquated) word glibbery, which pretty much meant the same thing, and glibbery is from Low German glibberig, also "slippery". Finally, it all derives from the Proto-Indo-European reconstruction gelh, which could mean "flourish", "green", or "yellow" (and is the root of words like chlorine, cholera, gold, and yellow). According to Google NGrams, the word glib was very popular around when it first started getting used and has since levelled off, now making up about 0.00004% of all English words.
Hello! I'm Adam Aleksic, a sophomore studying linguistics and government at Harvard University, where I founded the Harvard Undergraduate Linguistics Society. I also have disturbing interests in politics, vexillology, geography, board games, conlanging, and law.