The Collaborative International Dictionary of English v.0.48:

Megilp \Me*gilp"\, Megilph \Me*gilph"\, n. (Paint.)
   A gelatinous compound of linseed oil and mastic varnish, used
   by artists as a vehicle for colors. [Written also magilp,
   and magilph.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

megilp
    n 1: a medium for oil-paints; linseed oil mixed with mastic
         varnish or turpentine [syn: megilp, magilp]