subsesqui-

from The Collaborative International Dictionary of English v.0.48

Subsesqui- \Sub*ses"qui-\ [Pref. sub- + sesqui-.] (Chem.)
   A prefix (also used adjectively) denoting the combination of
   constituents (especially electro-negative and
   electro-positive bodies) in the proportion of two to three;
   as, a subsesqui acetate, i. e., a salt having two equivalents
   of acetic acid to three of the base.
   [1913 Webster]