on fire: to set a house afire.
aflame ( def 2 ).

1175–1225; Middle English; see a-1, fire Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
afire (əˈfaɪə)
adv, —adj
1.  on fire; ablaze
2.  intensely interested or passionate: he was afire with enthusiasm for the new plan

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

c.1200, afure, from a- "on" (see a- (1)) + fire (q.v.).
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Recently they set a school afire and then attacked the firemen who came to
  extinguish the fire.
He was still silent but began at once to erect a heap of dry sticks which he
  presently set afire.
Perfect for an underage, overeducated drinker looking to set the world afire
  with her sophistication.
We simply know that it will, and that the images it takes will set the
  curiosity and minds of people afire.
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