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.

Source: Webster's Revised Unabridged Dictionary (1913)