wiseacre \WY-zay-kuhr\, noun:
One who pretends to knowledge or cleverness; a would-be wise person; a smart aleck.
A wiseacre on the Oakland to Los Angeles shuttle this week said the next technological leap would be implanting cell phones into people's heads. He was kidding -- we think.
-- Chuck Raasch, "California is November prize for candidates", USA Today, August 24, 2000