(transitive) To pledge (something) as surety for a loan; to pawn, mortgage.
(politics, British) To designate a new tax or tax increase for a specific expenditure.
(Australia) To designate in advance the specific purpose of expenditure for government revenues (especially taxes).
"Levying customs duties and excises on necessities also ensured a relatively secure source of revenue. Revenues were generally hypothecated in an attempt to draw support from the public, for example funding an orphanage, gaol, hospital equipment and building works around Sydney." A brief history of Australia's tax system; Sam Reinhardt and Lee Steel; 04 September 2006