The Collaborative International Dictionary of English v.0.48:

Marginally \Mar"gin*al*ly\, adv.
   1. In the margin of a book.
      [1913 Webster]

   2. In a marginal[3] manner; barely sufficiently; almost
      not; as, marginally useful.
      [PJC]

Jargon File (4.4.4, 14 Aug 2003):

marginally
 adv.

   Slightly. "The ravs here are only marginally better than at Small
   Eating Place." See epsilon.