According to <i>Everyday American English,</i><font color="#ff0000"> sharp as a tack </font>means <font color="#008000">completely healthy</font>.<br/><br/>According to <i>Cambridge Dictionary of American Idioms</i>, it means <font color="#0000ff">very intelligent</font>. <br/><br/><i><font color="#226699">He may be old in years, but he's still as sharp as a tack and knows what he's talking about.</font></i><br/><br/><a href="http://idioms.thefreedictionary.com/sharp+as+a+tack"><a href='http://idioms.thefreedictionary.com/sharp+as+a+tack' target='_blank' rel='nofollow'>http://idioms.thefreedictionary.com/sharp+as+a+tack</a> </a><br/><br/>Which do you think is correct?