The Collaborative International Dictionary of English v.0.48:

Boot \Boot\ (b[=oo]t), n. [OE. bot, bote, advantage, amends,
   cure, AS. b[=o]t; akin to Icel. b[=o]t, Sw. bot, Dan. bod,
   Goth. b[=o]ta, D. boete, G. busse; prop., a making good or
   better, from the root of E. better, adj. [root]255.]
   1. Remedy; relief; amends; reparation; hence, one who brings
      relief.
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            He gaf the sike man his boote.        --Chaucer.
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            Thou art boot for many a bruise
            And healest many a wound.             --Sir W.
                                                  Scott.
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            Next her Son, our soul's best boot.   --Wordsworth.
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   2. That which is given to make an exchange equal, or to make
      up for the deficiency of value in one of the things
      exchanged.
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            I'll give you boot, I'll give you three for one.
                                                  --Shak.
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   3. Profit; gain; advantage; use. [Obs.]
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            Then talk no more of flight, it is no boot. --Shak.
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   To boot, in addition; over and above; besides; as a
      compensation for the difference of value between things
      bartered.
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            Helen, to change, would give an eye to boot. --Shak.
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            A man's heaviness is refreshed long before he comes
            to drunkenness, for when he arrives thither he hath
            but changed his heaviness, and taken a crime to
            boot.                                 --Jer. Taylor.
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WordNet (r) 3.0 (2006):

to boot
    adv 1: in addition, by way of addition; furthermore; "he serves
           additionally as the CEO" [syn: additionally, to boot]