Chambers1 gives "utter" as of two separable meanings (& according to Ayto2, from the same source): 'to the outermost reaches', ie "complete, total"; and 'to say out (express)', "speak, say or pronounce". Ayto, though more concise is, as usual, more interesting. He says of "utter":
English has two distinct words utter, but they come from the same ultimate source - out. The older, 'complete, thorough-going' [OE] originated as a comparative form of out (or ūt, as it was in the Old English period, and so morphologically is the same word as outer. It did not begin to be used as an intensive adjective until the 15th century. Utter 'express openly, say' was borrowed from the Middle Dutch uteren 'drive out, announce, speak,' a derivative of Old Low German ut 'out'.
1. "Chambers Dictionary of Etymology", 1988.
2. John Ayto, ""Dictionary of Word Origins", Columbia Marketing 1994.