My classics teacher supported the theory that the Greeks named other non-Greek tribes as barbarians because of the bar-bar sound of their speech. In Alexandrine times the saying of "all non-Greeks are barbarians" was used by all those who had classical education often as a put-down to the uneducated hoi polloi. However I do not think that the origin of barbarian had much to do with the beards of the non-Greek tribes as the Greeks themselves did grow beards, as one can see on the depictions of battles and heroes on the plethora of ancient Greek pottery and statues.