Refers to a sneaky hard-sell person who "talks out of both sides of his mouth" to make a sale --- i.e., he deceitfully "speaks two different languages" at stretegic times throughout da various settings during his sales-speech in order to flim-flam da naive buyer into parting with his moolah.
Encyclopedia Brown always knew dat "shifty" folks like Bugs Meany and Wilford Wiggins were chronically "buy-lingual" in their ongoing attempts to swindle other youngsters around Idaville, and so he had to always be on his toes intellectually in order to stay a step ahead of those hoodlums.
by QuacksO February 8, 2020