Source : Webster's Revised Unabridged Dictionary (1913)

Surtax \Sur"tax\, n.
   An additional or extra tax.

Surtax \Sur*tax"\, v. t.
   To impose an additional tax on.

Source : WordNet®

surtax
     n : an additional tax on certain kinds of income that has
         already been taxed [syn: {supertax}]
     v : levy an extra tax on; "surtax luxury items that cost more
         than $1,000"