Mythic dragon of Tolkien lore, featured in The Hobbit. Bred in Angband, took over the dwarf kingdom of Erebor in the third age, 2770. Occupied lonely mountain until Bard killed him with the black arrow. "Smaug" means "Squeezed through a hole" according to Tolkien's linguistics. This refers to the famous tunnel Bilbo discovered which is measured as 'five men high and three may walk abreast" and is Smaug's original point of entry into Lonely Mountain, but he can no longer get out.
Source: The encyclopedia of Arda, http://www.glyphweb.com/arda/