The Collaborative International Dictionary of English v.0.48:

Supertax \Su"per*tax`\, n. [Super- + tax.]
   A tax in addition to the usual or normal tax; specif., in the
   United Kingdom, an income tax of sixpence for every pound in
   addition to the normal income tax of one shilling and
   twopence for every pound, imposed, by the Finance Act of
   1909-1910 (c. 8, ss 66, 72), on the amount by which the
   income of any person exceeds [pounds]3,000 when his total
   income exceeds [pounds]5,000.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

supertax
    n 1: an additional tax on certain kinds of income that has
         already been taxed [syn: surtax, supertax]