Surprise, surprise, check out this other meaning:
http://en.wikipedia.org/wiki/Hypothecation
Hypothecation
The original use of the word hypothecation was for a pledge of property as collateral for a debt without transfer of possession to the party making the loan. The arrangement is common with modern mortgages - the borrower retains legal ownership of the property but provides the lender with a lien over the the property until the debt is paid off.
A more recent use of the word is as a contraction of "hypothetical dedication", as in a "dedicated tax" to be collected for a specific purpose. For instance, in the United States the Gasoline Tax (a tax on sales of motor vehicle fuel) is dedicated to the funding of transportation infrastructure....
So, to my surprise also, the word hypothecation does refer to taxes after all.