The Collaborative International Dictionary of English v.0.48:

Surtax \Sur"tax\, n.
   An additional or extra tax.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Surtax \Sur*tax"\, v. t.
   To impose an additional tax on.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "surtax":
   assessment, cess, conscience money, contribution, direct tax, duty,
   graduated taxation, imposition, impost, indirect tax, joint return,
   levy, progressive tax, separate returns, single tax, supertax, tax,
   tax base, tax dodging, tax evasion, tax exemption, tax return,
   tax structure, tax withholding, tax-exempt status, taxable income,
   taxation, tithe, toll, tribute, withholding tax