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]

WordNet (r) 3.0 (2006):

marginally
    adv 1: in a marginal manner; "marginally interesting"

The Jargon File (version 4.4.7, 29 Dec 2003):

marginally
 adv.

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