A loan repayment schedule in which the outstanding principal balance of the loan increases. 
To amortize a debt you kill off, reduce or extinguish the liability with each payment. With negative amortization the principal amount due, increases with each payment.
From c.1400 negare, deny  amortize
14c, deaden, extinguish, death. Extinguish a debt, is attested from 1864. 
Sumflow 01:53, October 23, 2010 (PDT)