According to J.K. Rowling, the last word of the book is "scar."
Possibly because the final paragraph is:
"Harry knew there was only one way to stop Voldemort from killing Upfish. He had to travel back in time. Back to the only time when Voldemort was vulnerable... "Temporus Reversi!" cried Harry, brandishing his wand over his head with a flourish. Sparks cascaded from his wand, big, magical sparks that suffused everything around him with a blurry, magical glow. Everything ran backwards around Harry, and the events of the past few days undid themselves before his eyes... Ron's marriage to Hermione and subsequent death, Snape's revelation that he and Dumbledore were the same person, Malfoy falling to his death from a conveniently-placed cliff, Ron's hypno-marriage to Bellatrix Gryffindor having recieved another million points for no Goddamn reason, all those magical creatures and objects with no real narrative value, Sirius Black's triumphant return from the Dimension of Death to kill 500,000 Death Eaters with an enchanted chaingun, Snape's revelation that he was a (magical) robot all along, even that one time the Death Eaters did something cool and totally evil offscreen while Harry was talking about Malfoy. All of this sped by faster and faster until it was just a magical blur, and then suddenly everything stopped. Harry was standing in an old-timey bedroom, looking down on a crib.* In that crib was a crying baby... a crying baby Voldemort! Harry steeled himself for what he needed to do. With a trembling voice, he incanted "Cru...cruciatus." And a bolt of magical green lightning spat from the tip of his wand, striking against the baby's forehead. But the baby was protected! Protected, dear readers, by love! Lo ho ho! A startling turn of events for our hero, for the spell that would have made short work of such a tiny member of God's kingdom of animals in the domain of Eukaryota bounces harmlessly off the baby's tiny, soft skull and strikes Harry straight in his! "Aargh," says Harry, as he disvaporates into a pile of ashes, ashes blown away on the wind and out the window. But we zoom in, only to see that on that most adorably evil forehead is now a tiny symbol, the symbol of Harry's ultimate defeat by the Dark Lord, he who can't be named, because he doesn't even have one yet, because he's a baby: a little finely-carved lightning bolt-shaped scar."
Also, I like Electric Six's Gay Bar. But you might have already known that, or at least suspected.
* I typoed "crab" instead of "crib," which would have made for a much funnier ending.